I have a 1974 2300. it backfires and shoots flames. i read in my clymer manual that if the diaphragm in the decel valve doesn't work it might mess with the idle and cause excessive hydrocarbons in the exhaust. i thought this might be the cause of the backfiring and the shooting of flames. my decel valve is missing and i wanted to know where you could find a replacement decel valve. thank you.
my 73 shoots flames (thru the carb) and backfires when the timing is way off... found that out when I was trying to time it.
did the same thing on my 77 when the timing belt jumped a tooth.
SRT told me once... "take the 'decel' (deceleration valve) valve off at the intake manifold, tap the hole into the manifold to IIRC 3/8 pipe insert a plug lfush and reinstall the decel valve. eliminates a potential vacuum leak and helps the engine rpm settle back down to idle after acceleration and between shifts." After SRT added that post I checked with a local mechanic and he said that it made sense...
So... if your decel valve is missing and blocked off as described by SRT it may not be the problem. hydrocarbons in the exhaust would be going out the tail pipe... not up thru the carb. I'm only guessing that when you said "shoots flames" you did mean thru the carb and not the tailpipe. I just can't see how that much unburned fuel would make it all the way to the tailpipe and ignite, I suppost it could but not in my limited experience.
If you do have flames coming out the tailpipe... I have no idea what your problem is and the experts on here will have to help ya ;D
it has a vacum cap on where i think it was supposed to go but it doesn't have a cap on the carb where it goes to the decel valve. it is shooting flames out of the exhauhst though instead of through the carb.
If it's shooting flames out of the exhaust, you obviously have a significant amount of raw gasoline somwhow getting through the engine without burning. It may either be a result of an excessive amount of gasoline OR an insufficiency of air, to the point where either will not allow complete combustion in the combustion chamber.
This can be caused by several different malfunctions, including many different ignition problems that can result in misfiring and the resultant unburned charge getting into the exhaust; fuel related problems including an overfilling fuel bowl (heavy float/mis-set float level/bad needle valve,seat combination/ too large jets, etc.), a fuel leak around the carb's throttle shaft, bad decel valve, etc.
Since you have homed in on the decel valve...
The decelleration valve is supposed to allow the throttle to close SLOWLY from a fairly wide open position, in order that the gasoline already having been sucked into the engine when the throttle was more fully open, would still have enough accompanying air with it and following it through to burn reasonably well. If the throttle snaps shut, backfiring can occur anywhere in the engine or exhaust, depending upon how much fuel is present and where it is when it ignites. If it continues to backfire in the exhaust, there is a strong possibility that you will blow out a pipe, muffler or worse.
If this IS a result of a bad decel valve, concentrating on easing off the gas pedal so as to allow the throttle to close more slowly will help considerably in alleviating the problem until you can get a replacement. (It may also help if the problem is timing related, so that isn't a sure identifier.) If the problem is actually carburetor related and results from having a continuous and excessive fuel flow, easing off the throttle may not help.
If you absolulutely cannot find a decel valve (doubtful), you may be able to rig an old style dashpot that actually PHYSICALLY eases the throttle linkage to a more closed position, rather than using the vacuum controlled decel valve. If you've worked on older cars this will be familiar to you. That is how most carbs used to deal with the same backfiring problem years ago, preceding the more tightly "designed", lower pollution emitting engines.
P.S. If the vacuum port on the carb is not capped, there is a vacuum leak. This will
not help the situation, as the odds are then strong that the engine's timing will be off quite a bit over at least a good portion of the engines' operating range, from the lack of proper (or any) vacuum advance. With less vacuum advance, especially
at higher engine speeds, there will be more unburned gas entering the exhaust to begin with, even if all the other engine systems are operating properly.
well. one thing sorry i forgot to mention that it happens when i let off of the gas not when i am on the gas. it does it pretty bad when you let off of the gas all the way at about 3000 rpm. then it doesn't pop so much when it goes down to about idle. so does anybody know where i can get a decel valve. i looked at napa online and they didn't have it. i think i need to talk to a store. thank you for the help.
On the off chance that what you are calling a decelleration valve is actually the check valve in the smog pump line, which MIGHT POSSIBLY also cause something akin to what you have described, I checked with Advance Auto, and although they didn't have a listing for a decel valve per se., that check valve part is their #779-2929.
This valve usually has an inlet and outlet size of about 3/4" diameter and is sometimes attached with a fitting to a steel line on one or both sides, other times it is inserted into rubber lines and with clamps securing it. If this one has a metal fitting which has to be unscrewed to release the valve, be prepared for that to be a problem.