Technical 704cc 'HOT' Engine Build

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Technical 704cc 'HOT' Engine Build

OK, I have attempted to start the carb tuning with item 1 & 2 from the last post. The first problem found was some funny airflow readings ie they didn't record the same each time :confused::confused: So I went back to my notes and found that I had not fitted a second seal to the carb venturis :mad::mad: It's important to note that any extra air getting into the the carb from the joints is a no no - you may as well burn fivers in a barrel hoping to put the fire out as tune a carb if you have a leak (n)(n) Initial airflow adjustments improved the running, but it's still very rich. Solex recommend that the idle jet will be 'in the ballpark' if the mixture volume screws are between 11/2 and 2 turns out, mine are both at 2 1/2 out so I'll set them at 2 out and try again. Once the airflow is recorded as constant I should be able to move on !!! More later (y)(y)
Ian.
 

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The main steps are as follows -
1. Balance the airflow between the carb venturi, this is contentious because the adjusting screws (Air Bypass) are set at manufacture and then blocked with wax.
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Ian.

Hi Ian,

If you've removed/refitted the butterfly valves (you've said in an earlier post that the throttle shaft is new), you may have no option but to adjust these air bypass screws to get the airflow into each venturi to become identical.

I've encountered these air bypass adjustment screws on some Weber carbs e.g. 40 IDF Downdraft Double Choke, and they are meant to be adjusted to achieve identical vacuum under the throttle plates at idle, they are not sealed on this carb model.

You mentioned in an earlier post, the option of using a Morgantune type of carb synchroniser to measure vacuum under the throttle plates at idle. Have you heard of carb. spacers/insulator plates which are tapped and have a pipe stub to which these carb gauges can be easily connected. These spacers were originally marketed to allow the fitment of a Vacuum Gauge (to monitor engine condition, air leaks etc. without having to remove and tap the inlet manifold. I can't recall who sold them, possibly SPQR/Paddy Hopkirk/Oselli Tuning/Gunsons? Perhaps you could drill and tap your existing carb spacer plate or fabricate/fit your own design?

Checking mixture at idle - gas analyser or Colourtune device (modern ways).

Old School - remove each spark plug lead (or short to earth) in turn, drop in revs should be identical if throttle opening/mixture at idle is identical across all cylinders. I suspect this won't work on a twin cylinder and might not be advisable to try if engine is fitted with electronic ignition...

Old school - allow engine to idle for 15 minutes. Remove and compare spark plug nose colours - should be identical if mixture is identical on each cylinder.

A vacuum gauge can also be used to indicate mixture strength but afaik it's not very accurate - better than nothing though? So if you connected up a Morgantune to balance air flow, you could then use it as a guide when adjusting the mixture volume screws?

Another thought re. balancing the air being drawn into each venturi.
Have you checked the cylinder compression pressures on this engine.

If the 2 cylinders are not producing the same compression figure, then maybe the two cylinders might draw different volumes of air, so could never be exactly balanced? I've always paid more attention to the initial pressure produced in a cylinder (1st compression stroke) than the final pressure achieved. Or then again, at idle, this might not make any discernible difference?

AL.
 
Greetings, More regarding carb manifold and fittings.
I have discovered that there is a potential problem with the carb manifold bolt holes in the Panda head in that they are rather shallow, the two inboard holes are the worst giving only 2cm depth at most, so some careful cutting of the bolts was carried out. I will say now that I have chosen to use socket head bolts as they give better room for manoeuvre above the manifold. But with the design of my manifold being cut from one piece of alloy I needed long bolts - finally requiring 2 at 100mm and 1 at 90mm.------------
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Ian.

Iirc, the usual recommendation for thread depth for maximum strength is 1.5 times the diameter - extra thread depth doesn't add extra strength. If you're worried, maybe the threaded holes can be drilled/tapped deeper - it's not like you risk breaking into the water jacket :) .If the threads do start to fail, you can always repair using thread inserts - Helicoil, Timesert etc.
Normally in this sort of application, studs with nuts would be used to attach the manifold, but I assume you're having to use cap screws for space reasons when fitting/removing the inlet manifold.


Just a thought - when changing Main Jets - would it be quicker/easier to remove the manifold complete with carb rather than removing the carb alone (3 easy to access cap screws versus 4 not so easy to access cap screws).

AL.
 
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News update, the engine is on it's way back from the balancing people (y)(y)(y)
Vibration Free at Bicester have done a stunning job with the balance figures improved beyond belief :eek::eek:
The following figures are from the official balancing certificate that is issued after the work is completed are all gm/mm.
Crankshaft - Initial 3290 final 20.1
Flywheel - Initial 719 final 17.7
Clutch - Initial 387.5 final 23.7
Pulley - Initial 505.9 final 6.9

So I think that I can call all that a Result !!!!(y)(y)
Once everything is back work will start on determining the pushrod length, support plate thickness, head gasket and squish band - more later.

Ian.
 

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further update re balancing, the engine has arrived safe and well and I've just had the first look at the work completed.
Something I forgot :eek: to add to the previous post was the results on the pistons and rods. Luckily they were all within tolerance thus -
Pistons Initial 404.7 gm within +/- 0.1 gm
Rods Initial 428.4 gm within +/- 0.1 gm

Once I had everything out of the box I took some pics of the work, The flywheel has been marked for TDC and both clutch pressure plate and the Pulley have been 'adjusted' by drilling to remove weight or by the addition of an alloy balance weight. I'll photo the crankshaft when I get at it. (y)(y) Needless to say it all will be cleaned before any other progress can be made. more later.
Ian.
 

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Hi Ian,
The engine is looking great. As I have got older and wiser* I have begun to understand that there are great gains to be had from engine balancing. When they balance the crank, do they take the oil mass within the crank into account?


I am hoping to start building my engine this year. I have had a set of barrels bored out to suit 80mm pistons to give 704cc. I have a panda 30 head and a set of twin carbs which are....unusual, but I'll start a post on that soon.


It's been great seeing your engine build story and others on here as it helps motivate me into getting started!




*allegedly
 
Greetings, apologies for the lack of progress, work has got in the way (n)(n)(n)
I have just been able to start the strip of the engine and have found more evidence of the balancing work. It seems that even the BIS crankshaft is too heavy when paired with my pistons and rods as there has been extensive drilling carried out to lighten it (see pic) I would have thought that the reverse would have been the case with weight added???? On the other hand my flywheel has only small drillings for the balance, even though it too has been lightened to 4.56kg.
I have found that the existing Crankshaft end float is now 0.05mm and it should be between 0.30 to 0.44mm so more work required in that area too :eek::eek:
More later
Ian.
 

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Greetings All, I've been having a hard time lately with the carb set up, In short, even though the engine started and ran well enough it was far far too rich (n)(n)(n)
In fact so much so that any other adjustment attempted made no difference to the rich running :mad::mad: SO methinks it's back to the books and revisit the initial set up.
It was only when I happened upon a French web site that I noticed something unusual :eek::eek: Now when I purchased the carb it came with all the required fittings (see pic 1) to connect the starter (choke) BUT on subsequent investigation involving the removal of the starter disc I found that the starter that was fitted didn't suit the fittings, so when I thought the choke was off it was in fact held fully on by the retaining spring :mad::mad::mad: So I've changed the fitting around to suit the starter disc and ......... it works!!!!!
(Pic2)(y)(y)(y) So the lesson learned there is to always check what you are getting and will it suit your application!!! I'm back to square one and now have a different initial result, in that it's now too lean at high revs and splutters slightly when accelerating from the progression circuit onto the main circuit. I've also broken the return spring :bang::bang: New spring on order!! So when I have more time I'll change my initial conservative jetting up a few stages and see where that gets me. The third pic shows all of the jets that are potentially changeable - top to bottom
Air Corrector and Emulsion tube (fixed)
Main Jet (reverse flow)
Pilot Jet
Starter Jet
Float Jet (with required 0.5mm washer)
A Starter Disc is to the side.
Back Later.
Ian.
 

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Greetings, having a lack of time is slowing things down at the moment, but I have managed to make the first jetting changes. Due to the now lean running and back firing at high revs I have changed the main jets up from 130 to 140. I have also changed the pilot jets from 45 to 43 to either make the spluttering I mentioned previously when accelerating from progression to main worse, or to see if leaner jets are required. It's a bit like eating something you've not tried before to see if you do like it or not ie "the proof of the pudding is in the eating" (y)(y)(y)
The one thing I dislike about the Solex C35PHH is the position of the starter and main jets, the carb must be removed to get at the access plate on the bottom. Some Solex carbs don't have this arrangement, but they are much larger and heavier. Weber and Dellorto are better in this respect but again are very expensive no matter how you obtain one as the part cost is high. I'll report back when time permits.
Ian.
 

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Greetings, had a quick run yesterday and have found the following -
engine still running lean (though not so bad) and shows slight spitting back through the carb so my change of Pilot Jets has compounded the problem. The next change is to richen up the progression circuit again and change the Pilot jets back to 45 from 43. At the moment I'm happy with the Main Jets but they may show lean running when driving so I am prepared to go up another stage if this is the case and see what the result is. (y)(y)(y) More later.
Ian.
 
Ian, would it not be cost-effective (and save you a pack of time) to put your car on a rolling road? You have one 12 mile up the road from you---"Daytuner" in Ripon Road, Harrowgate. I had my Dellorto FZD set up on my local rolling-road---saved me a lot of hassle and time.
 
Ian, would it not be cost-effective (and save you a pack of time) to put your car on a rolling road? You have one 12 mile up the road from you---"Daytuner" in Ripon Road, Harrowgate. I had my Dellorto FZD set up on my local rolling-road---saved me a lot of hassle and time.

Hi Tom, Yes, I agree going straight to a rolling road would end my trial and error, but I am enjoying getting to the bottom of this rather dark art :eek::eek: Once I am at a stage when the engine is running and driving well - ish will I invest my hard earned in a visit to the rolling road. My engineer man Alistair has recommended a shop in Northallerton that is " sympathetic" with older engines and their hourly rate is less too (y)(y) I have also the prospect of completing my new engine build when It gets back from Alistair too. It's all going to be a spring into summer project - time permitting (y)(y)
Ian.
 
Greetings, I finally managed to get some time on the car today and have made some progress, but not as much as I thought at first (n)(n) After going up one stage on the Main Jets to 140 and the Pilot jets to 45 I warmed her up checking for smooth-ish running and decided to take a quick run in the sun,(y)(y)
I can report that it seems to be driving well at this stage, with acceleration and moderate speed being achieved easily. But, (there's always a but!) I am getting an indication of a vacuum leak at idle!!! Whilst trying to maintain carb butterfly position almost closed to inhibit the progression holes working, the idle slowly reduces to almost nothing and the engine will cut out eventually. So, I've initially gone up one more stage on the pilot jet to 46 and adjusted (increased) the mixture to 2 1/4 turns open to richen the idle circuit, and made sure that the vacuum sampling ports downstream of the butterfly are blanked off and the Vacuum advance port is blocked off too. I then ran out of time :mad::mad::mad: So it's driving - in fact it accelerates like a scalded cat :devil::devil: I'm getting there - slowly. The second pic shows all the vacuum ports in place. More later.
Ian.
 

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Greetings, today sunshine so time to grab some more diagnosis (y)(y)
The car still starts well although the choke cable needs fixing to operate smoothly,
Once warm it was obvious that the previous adjustments to the mixture and larger Pilot jet seem to have improved things at idle (y)(y) But, the idle overall is still not good with the engine just running at 800rpm. Still, it does not cut out anymore (y)(y) I did have an Epiphany though - test for a vacuum leak with some brake cleaner :eek::eek: (do it in the open because it's nasty stuff) with the rpm set at 1500 I sprayed said cleaner around all the carb and manifold joins - nothing !!!! :D So I'm happy that I've sorted that potential problem. I think overall that I'll readjust the mixtures again and go up one more stage with the pilot jet if required.The pic shows the carb as it is now with the vacuum points plugged and blocked with silicone More later.
Ian.
 

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Greetings, finished the garden early so time for another diagnosis run (y)(y)
Engine started well although the choke cable still needs sorting so that I can get the choke off without getting into the engine bay !!! (n)(n)
I used my timing light to set a good 800rpm tickover as the gauge needle fluctuates, and then went for a short-ish drive. It drives well but the idle is still suffering so more adjustment needed. The mixtures are now at 2 3/4 turns open and the good book of Solex words :)worship::worship:) indicates that I should go up another stage with the Pilot Jets so that means up to 47 on both. I'll then have to reset both mixtures from the start again and see what transpires. More later.
Ian.
 
Greetings All, off today so decided to sort the jetting out before the rain !!!!
I've changed the Pilot Jets up one more stage to 47 on both. The mixtures were slightly more difficult this time and I've got satisfactory running with them both set at 1 1/2 open. There is a possibility that things are still slightly too rich as when accelerating slowly there are signs of rich 'hunting' before the main circuit cuts in. But, I am satisfied that without better equipment and a rolling road I will not be able to get things exact (n)(n) I had to increase the tension on the return spring to ensure that the carb butterflies close fully at idle and the accelerator cable has been adjusted to give the linkage a chance to achieve this. Overall this engine (712cc) still fluctuates a little at idle and this may be as a result of the cam and timing, with the larger carb setting too. So to summarise the set up at the moment prior to a rolling road set up is as follows -

Carb - Solex C35PHHE 10
Venturi - 31mm
Main Jets - 140
Air Corrector Jets - 135
Pilot Jets - 47
Float Jet - 1.3
Starter Jet - 90
Both Mixtures set at 1 1/2 turns open

The choke cable still requires work but is functioning at the moment (sort of) :eek:
I will continue with the new 'Hot' engine build as soon as the unit arrives back from the engineer (y)(y) .
Ian.
 

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You must have endless supplies of carb jets Ian......Oh......and of patience![emoji16]

Oh yes Peter, bags of both - I was not going to let this rather black art defeat me (y)(y) It has also taken a lot of rather boring reading to actually visualise what does what when you place your plates of meat onto the gas !!! I may take up giving talks about the atomisation of fuel, intake velocities and the relationship between Jets of differing sizes and the efficient combustion of the fuel air mixture - and then maybe not (y)(y)
Ian.
 
Greetings, Update from the engine man, they are having some trouble getting the correct squish band and staying within my specified maximum Compression Ratio of 10.5:1 Given that the Panda head I wish to use is now at 34.2cc the figures need to be adjusted by changing the deck height. The spigot support plates I have supplied are 1.0 and 0.6mm, I have one more that is of the correct spigot radius of 85.5mm but the thickness is unknown (n)(n) Anyway, when we arrive at the correct squish band of 1.0mm (including the head gasket of 0.5mm) the figures give a C/R of 11.0:1 I don't wish to go this high, but given that the Cam is of 300deg duration (40/80-80/40) the overlap may mean we have to accept a higher C/R to make the engine/cam work well. There is a possibility that my man will have to Dish the piston top to give slightly more capacity if the present figures don't work, (this will give better swirl in the cylinder) The figures are still in the mix at this time.
It is all dependant on the fuel quality and Ron (octane) number too, if you wish to use a tweaky cam you must use good fuel of at least 98-99Ron. I use Tesco Momentum with Millers additive to increase the octane rating by 2 points and for ethanol protection. Super Racing fuel is available but very pricy. My man is also checking the rocker angle against the cam lift and pushrod length and if the pushrods need to be cut to suit so be it. My present engine has the pushrods cut by about one cm to suit the cam and rocker angle. The final work being done before I get my grubby hands back on it is the fitting and timing of the timing gear set mentioned earlier, more on that when finished.
Ian.
 

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