engine back fireing

60

Asked by Mar 11, 2011 at 04:16 PM about the 1968 Volkswagen Beetle

Question type: Car Customization

Hi, I have just fitted a reconditioned engine in my 68 Bug. When I rev the engine it starts back fireing in the silencer. the engine is a 1600cc with a counterbalanced crank high lifters 1.4:1. any ideas please thanks to all.

4 Answers

1,705

could be several things. first check for exhaust leaks. this is the most common for backfires in the exhaust. second would be probably carburator jetting. if it's running very rich you can get some popping on deceleration. exactly why did you decide to use 1.4 rockers? what are the other engine specs as far as cam, heads, and carbs? did you build this engine, or somebody else build it for you?

60

Hi thanks for answering. yes my son decided on making a turbo on a 1600cc engine. the specs are: engine is standard as far as anything else goes just the rockers and a counterbalanced crank, light flywheel. we are runnning it on a twin carb 26 and 28 pct. but now we are running it without the turbo because it was miss firing. without the turbo it backfired at 4500rpms. yesterday to tell you the truth he changed the coil and the distributor and it seems that it is working better just a bit rough wich in my opinion is because it is a bit retarde. Do you think it should run with the turbo and with the specs I said earlier. we are not looking at anything special just a little push. thanks and regards. Charlie

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1,705

well you run into alot of different problems when you turbo one. for starters, when you add 1.4 ratio rockers, you have to change the pushrods. what i mean is that adding these rockers will make your rocker geometry different, and it needs to be set. most of the time this involves cut to length pushrods, shimming the rocker stands, or both. if this is not done, and done right, you will run into problems with the valves not closing fully, melting valves, loss of compression, all sorts of things. this is the first thing to check. what is the compression ratio of the motor? adding a turbo is nice, but if your compression is to high you will run into detonation and this will kill the motor very quick! next, if the motor is running the stock crankshaft, your rpm limit should be 4500 rpm anyway. anything over that you are going to start pounding the main saddles of the motor and basically destroy the case. what carburator are you running WITHOUT the turbo? single or dual carbs? when you go turbo you really need to work on the ignition system. you can run a mechanical advance distributor (like the 009) but you need to either lock it to a certain advance (not real good) limit the advance (a little better) or run something like a msd distributor with box and boost retard (alot better) or go with a crank fire edis style( best).

60

hi, yes the valve geometry is fixed. the crank is a counterbalanced one and the compresion is a standard one a 7.3:1 which should be good. the carb without the turbo we are using a twinchoke with the chokes being 26mm and 24mm we used a standard distributer which was locked by a friend of mine but it backfired withn it. but any way now we got ourselves another chassis so we decided we will try it on this chassis to see its output. I whish i can use a set of 45 weber . but still i need to see it on a dyno. thanks mate

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