Gents...I was brave and took my little 1970 500F out on the highway here in NY and was cruising at a good clip of about 100kph for about 10 miles straight. When I let off the throttle, the engine is backfiring but through carb, not the exhaust. I have a reconditioned head with new valve seat, new valves and a new carb. Is the timing off? Is it running too lean? Tappets not adjusted correctly? Other than that it was running like a champ.

Your 500 does 60mph? WOW!

(just messing )

Probably best to check to valve clearances (lash) and ignition timing first before messing with the carb. With a new head and valves, valve clearances can close up fairly quickly as things bed-in.

Also, after the above run, would be a good time to pull the spark plugs and see what colours you have on the plug tips. Ideally should be straw to light brown colour if mixture etc. is correct.

Al.

F123C said:

Your 500 does 60mph? WOW!



(just messing )



Probably best to check to valve clearances (lash) and ignition timing first before messing with the carb. With a new head and valves, valve clearances can close up fairly quickly as things bed-in.



Also, after the above run, would be a good time to pull the spark plugs and see what colours you have on the plug tips. Ideally should be straw to light brown colour if mixture etc. is correct.



Al.

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Yep! Here's proof.



I will follow your advise and check the clearances again, timing and plugs.

Perhaps before re adjust/check the valves it will be wise to retorque the head maybe if you are using gasket and have done some miles? Usually retorque is taking place after 1000 km's, Please report back.
Thomas

Labman said:

Yep! Here's proof.

View attachment 184695

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That's only 99 kph!

Are you sure the 'backfire' you experienced after running your 500 at 100 kph for 10 miles wasn't just a 'sigh-of-relief' from your little engine when you finally backed-off?

Al.

F123C said:

That's only 99 kph!



Are you sure the 'backfire' you experienced after running your 500 at 100 kph for 10 miles wasn't just a 'sigh-of-relief' from your little engine when you finally backed-off?



Al.

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Haha! Actually I was running at 100kph for a while then when I took the pic it was at 98/99kph [emoji4]

gordinir8 said:

Perhaps before re adjust/check the valves it will be wise to retorque the head maybe if you are using gasket and have done some miles? Usually retorque is taking place after 1000 km's, Please report back.
Thomas

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I did retorque it once already, but I did it earlier on. I don't think I put even 500km on it yet since the head work. I will do it again and see.

If it was a new or refurbished head/valves etc., I would treat the engine somewhat gently for a while longer, because I think that all of these parts deserve to have a running-in period. With a new engine you are advised not to go past 38mph in 4th gear for the first 450 miles.

fiat500 said:

If it was a new or refurbished head/valves etc., I would treat the engine somewhat gently for a while longer, because I think that all of these parts deserve to have a running-in period. With a new engine you are advised not to go past 38mph in 4th gear for the first 450 miles.

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Yes. I thought it was a bit aggressive taking it on the highway for so long at that speed. But it was running so good I really wanted to see how well it ran for a longer distance.

Labman said:

it was running so good I really wanted to see how well it ran for a longer distance.

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So I re-torqued the head and it was fine. The tappet closest to the rear of car was way off. Lots of play. Readjusted all off them to 15mm. Now just need to get a new valve cover gasket to put it all back together. I will post again when done.

I sincerely hope that you have adjusted the tappets to 0.15mm (15mm sounds a mite wide!). I would recommend that if you can get it, you use a 'ruberoid' rocker-cover gasket, rather than the cork variety. Also, don't forget to put a new fibre washer UNDER the plain washer (that goes under the nut).

the hobbler said:

I sincerely hope that you have adjusted the tappets to 0.15mm (15mm sounds a mite wide!). I would recommend that if you can get it, you use a 'ruberoid' rocker-cover gasket, rather than the cork variety. Also, don't forget to put a new fibre washer UNDER the plain washer (that goes under the nut).

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Haha! Yes. I meant .15mm. New rubber type gasket used and very true about the fiber washer under the plain washer. I went crazy with oil leaking through those nuts before I figured out they were missing. Now the road test as soon as the weather breaks. Any tips on correct timing? I have a 123. Is it just set it and forget it or should I still use a timing light?

Made a mistake. Not backfiring from carb but from exhaust. No matter what I do I can't get it to go away. I'm ordering new plug wires as I got a shock when I touched the plug end at the coil. I don't think that should have happened.

Backfiring is a timing issue. If you've got a 123 ignition I would get a timing light on it.

Agree with Paolo--get yourself a timing light, and check the timing, adjust as required. You cannot set up 'electronic' ignition statically, you MUST use a timing light. My engine occasionally back-fires through the exhaust, but it is the cam (35/75/75/35, in a well tuned engine).

the hobbler said:

Agree with Paolo--get yourself a timing light, and check the timing, adjust as required. You cannot set up 'electronic' ignition statically, you MUST use a timing light. My engine occasionally back-fires through the exhaust, but it is the cam (35/75/75/35, in a well tuned engine).

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I have one. Just a basic one. Two timing marks aligned during timing. Should it be advanced or retarded? I have done both and still backfires. By the way it backfires not during acceleration but at coasting when letting off the gas.

My engine pops through the exhaust on the over-run, a bit more when the engine is hot than when the engine is cold. As well as timing, this could also be caused by running a slightly too rich carb mix. However, with an air-cooled engine, I would rather run it a bit rich than a bit lean---less chance of a damaged piston. Have you thought of setting the timing absolutely spot-on, giving the engine a good run, turning it off as soon as you get back home (i.e don't let it sit idling) and when it is cool enough to do so, whip a plug out and check its colour.

the hobbler said:

My engine pops through the exhaust on the over-run, a bit more when the engine is hot than when the engine is cold. As well as timing, this could also be caused by running a slightly too rich carb mix. However, with an air-cooled engine, I would rather run it a bit rich than a bit lean---less chance of a damaged piston. Have you thought of setting the timing absolutely spot-on, giving the engine a good run, turning it off as soon as you get back home (i.e don't let it sit idling) and when it is cool enough to do so, whip a plug out and check its colour.

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I have experimented with a couple of different plugs but makes no difference.
Which way is more rich with the fuel mixture screw....in or out? I have run with it all the way in and still backfires.

I think that you will find that the correct spark plug for the 500 is a:-- NGK BPR7HS (or the 'other manufacturer' equivalents). A change of plug won't rectify your 'fault'. The important thing to ascertain is the plug COLOUR after you have given it a good run, as per my previous suggestion. By inspecting the colour of the plug you will be able to ascertain, to a degree, whether the engine is running rich or not. Adjusting the IDLE mixture screw won't effect it either. You may have to change the main jet (which is a 'carb off' job). The correct MAIN jet for a 500 (all variants of the 26IMB Weber, with the exception of the IMB3 which was used on the 'Sport 500') is a1.12mm jet---for the IDLE jet, a 0.45mm jet. :bang: