Well I got the engine out of Barry and changed the clutch, put it back in and he ran and drove fine on the short drive I took. But I still had a few bits to finish off so yesterday I adjusted the clutch and a few other minor things but then he wouldn't start. He has been getting progressively harder to start when cold recently so I pulled out the plugs and found them to be black. I cleaned them up and he started easily but was running like crap and didn't like the choke at all. I checked the valve clearances recently so I'm thinking it might be the carb.
However, the carb fitted is an FRG28. I have stripped down an weber 26 IMB before but I know nothing about this unit or where all the relevant jets are. Does anyone have a schematic by chance?

Andrew:---does the engine start OK from stone cold? If it does, but gets harder to start as it gets warmer I would initially look at 2 things---(1) check that the float is not punctured and slowly filling up with petrol and. (2) check that the needle valve is operating correctly and not sticking open. Both of these faults will make an carb run rich

the hobbler said:

Andrew:---does the engine start OK from stone cold? If it does, but gets harder to start as it gets warmer I would initially look at 2 things---(1) check that the float is not punctured and slowly filling up with petrol and. (2) check that the needle valve is operating correctly and not sticking open. Both of these faults will make an carb run rich

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Tom, it's actually the opposite. It is hard to start cold and hesitates a lot until it is fully warmed up at which point it runs fine and starts fine, or at least that was what it was doing. Now it's hard to start and runs poorly all the time except at idle.
I will have a rebuild kit for this carb later in the week so I'll strip it down and check the float and needle, although I'm not sure the kit comes with a float.

The other option will be to replace it with a Weber 26IMB I suppose. Any idea if the Dellorto has a performance advantage over the Weber?

As far as I am aware Andrew, the Dellorto was just an optional carb for the 500 range, but it might have a slight performance enhancing ability. ALL my information on the 500s, which includes a pukka 'factory' work-shop manual, only (with the exception of the Giardy which had a 26OC carb) lists and describes the IMB range of carbs. When Abarth used a 'little' carb, as on the '595', they fitted a Solex, the '28IB2' (the 595ss and the 695 used the Solex 34PBIC).
If you do revert to a Weber carb, try and lay your hands on the carb off a 650 126 (Mk2 version)---the Weber 28IMB---slightly bigger carb, slight gain in performance.

the hobbler said:

As far as I am aware Andrew, the Dellorto was just an optional carb for the 500 range, but it might have a slight performance enhancing ability. ALL my information on the 500s, which includes a pukka 'factory' work-shop manual, only (with the exception of the Giardy which had a 26OC carb) lists and describes the IMB range of carbs. When Abarth used a 'little' carb, as on the '595', they fitted a Solex, the '28IB2' (the 595ss and the 695 used the Solex 34PBIC).
If you do revert to a Weber carb, try and lay your hands on the carb off a 650 126 (Mk2 version)---the Weber 28IMB---slightly bigger carb, slight gain in performance.

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Thanks Tom. Is the 28IMB a simple swap or is there some mods required?

I have got the FRG28 off and the base is warped so I'll flatten that off and fit the rebuild kit. The float and needle valve seem to be fine but hopefully the kit has some instructions, such as float setting, otherwise I'm flying blind.
This being a 28mm throat carb (I hadn't realised despite the clue in the name) I noted that the spacer/tray under the carb is probably a 26imb type and has a 26mm hole. Will this be restricting the performance of the carb or will it make no difference as the throat size on the head looks pretty small (standard 500 engine)?

The 126 tray that goes under the standard Weber 28IMB carb has a 30mm bore.

Toshi 975 said:

The 126 tray that goes under the standard Weber 28IMB carb has a 30mm bore.

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Thanks for that. I did a quick measurement of the bore into the head and it's 26mm so i guess this will restrict things anyway?

I got the rebuild kit which has no instructions. But it's just some gaskets and a needle valve and holder so self explanatory...apart from a very small round rubber seal- which I couldn't find a use for- and no measurements for the float calibration, so i left it as it was. Today I'll see if it'll run

I would have thought that fitting that carb to a 500 would have been part of a tuning "package" involving cylinder head, camshaft, exhaust as well as the carb. The Dellorto is a nice item to have but if it is just bolted onto a standard engine it will achieve very little apart from using more fuel.
As part of the tuning on my 499cc engine I had the head ported including opening the inlet to 30mm and larger modified valves fitted.

Toshi 975 said:

I would have thought that fitting that carb to a 500 would have been part of a tuning "package" involving cylinder head, camshaft, exhaust as well as the carb. The Dellorto is a nice item to have but if it is just bolted onto a standard engine it will achieve very little apart from using more fuel.
As part of the tuning on my 499cc engine I had the head ported including opening the inlet to 30mm and larger modified valves fitted.

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It was just what was fitted when I got the car but maybe a head upgrade to match the carb might be a good idea in the future. Right now I just can't get the car running:bang:

Fayray said:

I got the rebuild kit which has no instructions. But it's just some gaskets and a needle valve and holder so self explanatory...apart from a very small round rubber seal- which I couldn't find a use for- and no measurements for the float calibration, so i left it as it was. Today I'll see if it'll run

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Well that was a 'spoke too soon' moment. With no instructions and not knowing which way up the needle valve went into its holder (it fell out on disassembly before I took note) I firstly got the new valve the right way up but the carb just flooded and spewed out petrol and wouldn't fire. I then put it in the other way and it starved but did start and idle. I then noticed the new valve was slightly different to the old one and had a longer travel on the plunger so reverted back to it but put it in the wrong way. The car would drive 30m down the road then starve.

While all this was happening I wasn't sure it was the carb so checked everything and even fitted an old 26 IMB 4 in rough condition that I had lying around to confirm it was the carb and not ignition. It ran poorly but seemed to confirm I was looking in the right place.

I put the Dellorto back on with the valve the wrong way up (I still hadn't worked it out) and, amazingly, still had the same starvation symptoms. Then I removed the carb for the 5th time and put the needle in the correct way - as it turns out - and also noticed that the wee plunger sits in a little tab on the float. He now runs fine. So I took a small problem (a bit of crud in a jet I'm guessing) and turned it into an epic problem that literally kept me awake at night.

Oh for an exploded diagram of this carb over the last few days. Still, I'm documenting it here in case someone else has a similar issue. The rebuild kit ($60 or 30 quid) was a handful of gaskets, one of which was unique to this carb, and a needle valve and holder that didn't seem to work.

Photes: On the left is the correct way, the right is the incorrect way. Apologies to those that think that is stunningly obvious:bang:

I think that your carb is a pretty rare item in fact although I have heard of them I have not heard of anyone else running one. When I saw your picture I looked at the spare Dellorto FZD one I have and they are identical which makes sense. I googled the carb and in images there is an exploded diagram of a Dellorto FRD28 which may be very similar if you look at it. The float looks the same as the FZD as well. You have to hook the valve pin onto the slot in the float on assembly as far as I can remember.

Thanks Toshi, I did come across someone else running one on here and I also came across one locally for sale recently by a 500 used parts supplier. My car being a '73 500F I had wondered if it was a late production factory fitted part but now I'm thinking that's not the case.

That FRD28 diagram would've been useful. I looked up other FRG models but didn't look for FRD. However, it doesn't show the clever way the valve pin hooks onto the float and that was what was tripping me up. I solved the issue when I accidentally hooked it onto the tab.
Ultimately, as you pointed out earlier, this carb won't be working to its potential and just using more petrol. So maybe I should either consider some head etc work to accommodate it or change it for a 26 IMB.

I think that the Dellorto carb is a good item to have in your locker especially if it is in good condition. You could look out for a 126 drip tray/spacer might be of benefit in the future depending upon your plans.
You said that you had a 26IMB carb. I have lost count of how many of these I have fixed/rebuilt and you can sort most things if the main parts are not broken. What is the problem with your 26IMB?
As for head tuning which is beyond me I went for a racing motorcycle engineer who after a thorough inspection of a standard 499cc said that he was confident of getting at least 2hp per cylinder out of the engine after head modification. I know you have guys down there as I have seen the film , "The Fastest Indian"

My 26IMB is an early D model and the top is warped. I read this was a common issue with the 26IMB 4. Can this be fixed? I figured a 26IMB 6 from an F would be relatively easy to find here and refurb.
I took the car for a 20 minute drive yesterday and it went well until I turned into my driveway and he died at idle. Seems I haven't got that bit of crud out after all:bang:.

I've seen the film, and loved it, but I didn't see any of myself in Burt Munro, sadly.

So is your car a 500F?
I have straightened the tops before making careful use of my big vice along with other bits & strategies. Some get scrapped by bodged up drilling of the threaded holes for the inlet elbow studs and excessive wear on the float pivot posts is a tricky one to deal with.
The top on the 26IMB6 is a little more stable as on top stud goes through and screws into the carb body.
The bottom flanges are very prone to warping and high mileage carbs suffer from worn butterfly spindles.

Thanks Toshi, it is indeed an F. A friend has a 26IMB 10 that I can buy so I may go that route, give it a good clean and check over and keep the 4 and the FRG as spares.