Backfire through carb

[horndog]

I have a 2-bbl 120hp mercruiser, it is a 1978, and the carb is a rochester. I figured it was one of two things maybe even both: timing way off or the carb is too lean.<br /><br />I just got done rebuilding the carb, and still the same problems. As far as the timing is concerned, well, this is how I marked it...<br /><br />turned the engine over by hand till #1 cylinder is at TDC and the rotor is pointing to cylinder #1 on the distributor. As far as how close it is, I have no clue, its as far as I can get it. marked that spot, which should be TDC or 0 degrees. <br /><br />I set it to 8 degrees BTDC, and it runs like mud, fiddled with the idle mixture screws and that just made it worse. <br /><br />Set the idle mixture screws till the rpms got high then started to burp and backed it off till its high again, then turned it down through the idle stop screw. <br /><br />Did I do everything right? I getting awefully frustrated!

 

[22WRF]

Re: Backfire through carb

Are your sure it was on the compression stroke when you set it @ TDC?

 

[DJP]

Re: Backfire through carb

DELCO IGNITION TIMING CHART<br /><br />ENGINE INITIAL TIMING TOTAL TIMING<br />CID @ 500 RPM @ 4000 RPM<br /><br />181 0 deg 24 deg<br />250 10 deg 32 deg<br />262 2 deg 24 deg<br />305 8 deg 26 deg<br />350 8 deg 25 deg<br />454 12 deg 30 deg<br />552 12 deg 30 deg<br /><br />Warning: These timing Numbers should be verified with your owners manual. These are not necessarily true for every engine setup, and variances can occur. Use these as a reference base only.<br /><br />Check total timing. This must not exceed 24 degrees at 4000 RPM for 181/262 CID engines, 26 degrees for 305/350 CID engines and 30 degrees for 454/502 CID engines.

 

[Juancho]

Re: Backfire through carb

I have pretty much given up on statically timing old engines. I just set them as far advanced as they will go without detonating. I still use my timing light just to make sure I am close to the factory setting. <br />I rebuilt a motorcycle once that had the same problem. It turned out that the cam gear was off by a couple of teeth. If you have an overhead cam with a timing belt, maybe your timing belt has slipped a couple of teeth?

 

[horndog]

Re: Backfire through carb

trippin, my motor is a 153CUID, not 181, that would be a 3.0L, mine is a 2.5L<br /><br />I am not sure it is the compression stroke, any way to tell if it is the compression stroke?<br /><br />I should check the timing chain or belt to make sure it is still lined up properly...<br /><br />I have also heard it may be a burnt intake valve as well.<br /><br />I did "listen to the drive train and didn't hear anything bad, just the nice calm "swoosh" sound, almost put me to sleep it was so smooth. Anyways, I don't know if a burnt valve would sound any different. <br /><br />I wish i had a compression tester

 

[DJP]

Re: Backfire through carb

The easiest way to discover when the #1 cylinder is at top dead center is to remove the spark plugs and rotate the engine by hand simutaneously keeping your index finger in the spark plug hole of #1. Once you feel pressure start to build then you are on the compression stroke, then you just line up your marks. However If the motor runs now and only back fires, more than likely It's just gonna be a timing issue. If the boat were mine, I would re-set the distributor at TDC for #1, snug the housing just enough to allow you to move it by hand and start it either in the water or with the muffs. Let it warm up and adjust the timing 2deg ATDC, throttle up, and back checking for backfire, continue in increments of 2 deg advance, until engine runs out smooth, with a clean idle. Throttle up and back, then retard timing 2 deg, and check for back fire again. Depending on the MFG timing recommendation the prior timing procedure may have to be done on the retard slope rather than the advance slope. But try the advance slope first. You can do this without the aid of a timing light if you got a good ear. Have you checked the Dwell in the distributor ? I'm assuming you do not have an HEI system. you may have to re-adjust your carb after you resolve the back fire issue as adjusting the mixture screws will cause it to run leaner or richer. The rule of thumb for the mixture screw(s) is, without the engine running, screw the mixture screw(s) all the way in and then back out 3 full rotations, and back in 1/2 rotation. Final adjustment can be made later once you have the engine running smoothly by checking the condition of the plugs. Oh and by the way I think your engine has timing gears rather than a chain, so that wont be your problem.

 

[DJP]

Re: Backfire through carb

OK forget about dwell and points, cause you shouldn't have em. If you have the conventional ignition...Distributor rather than a digital ignition then try the afore posted process. If it's a digital ignition then, take it to your local dealer. Sorry that's all I can figure. Good Luck

 

[horndog]

Re: Backfire through carb

ok, took your advice, and it runs much better, but I have no idea what it is suppose to run like. If I goose the throttle hard, really hard from in gear to WOT, it will say, "WHAT??" and cough and die, but if I take it nice and hard, it will have a little flat spot around 2500 rpms, but other than that its fine.<br /><br />Just don't know how it will behave on the water with a load.

 

[DJP]

Re: Backfire through carb

Ok, it's sounds like your on the right track, now mabey a modification of the air / fuel mixture screw. Come back out a half a turn on the screw, and try the jump to light speed again. If you have to come out more than 1 turn to accomplish what you want then another 2 deg adjustment in the ignition is needed, in the same direction you went previously.

 

[horndog]

Re: Backfire through carb

i have two screws, so back them both out right? <br /><br />I figured it would take a lot of "playing" around. I will try it on monday and see what it does. Hopefully I finally get it. Geez, what a pain!<br /><br />All I want to do is enjoy it!

 

[DJP]

Re: Backfire through carb

Question is.... Does your carb look like the following ? This one being a Mercruiser carb.<br /><br />

 

[DJP]

Re: Backfire through carb

Or this one.......Being a Rochester<br /><br />

 

[horndog]

Re: Backfire through carb

its definately a rochester

 

[horndog]

Re: Backfire through carb

is there a difference in tweaking these trippin? <br /><br />Why you asking? Is there a big difference between the two carbs? As far as I know (from Seloc) the only difference is that rochester has two idle mixture screws and the mercarb has one.

 

[DJP]

Re: Backfire through carb

Just trying to determine where on the schematic breakdown both of your screws are. Can you Identify for me both of the mixture screws ?

 

[horndog]

Re: Backfire through carb

they are at the base of the throttle plate. That schematic is turned around compared it being on the engine. On the engine the screws are on the port side of the throttle plate at the base

 

[DJP]

Re: Backfire through carb

I can't picture 2 screws, but if you say so. Yes screw them both in and then back out 3 turns and 1/2 a turn in, then adjust by 1/2 turns, poor response means adjust out, backfiring means adjust in. That's about all i can do without actually being there. Good luck with it.

 

[horndog]

Re: Backfire through carb

I also noticed a i measured the float drop wrong. I measured it from the middle line on the float, not the bottom of the float. Would this make a difference?

 

[DJP]

Re: Backfire through carb

Measuring the float is a judgement of where the needle valve tab engages the needle valve tip and how far the travel of the needle valve is in relation to the float in order to cease fuel flow upon closure. The idea is to maximize fuel quantity within the float bowl, and simutaneously prevent flooding of the carburation system. I guess I would suggest that you observe the level of the float at full open, then adjust the needle valve tab so that you know you have definite closure upon actuation. If your carburation system is not flooding itself then that would be the least of my worries unless your confident the float is set so that your engine is starving for fuel.<br /><br />I'm afraid that the information I am providing may just be confusing you. Perhaps another post by you would be appropriate, whereas I won't interfere. Someone else may be able to explain without so much theory.

 

[horndog]

Re: Backfire through carb

its not confusing, I am just tring to eliminate one thing at a time. <br /><br />After reading the manual, the pump rod is too long thus not compressing based on my measurements, it is 1/4" too long. That maybe a problem. I adjusted the float, its not way off, but now its to spec. I also noticed the accelerator pump gasket(the rubber part) was chewed to chit, so that maybe a big problem related to the pump arm. <br /><br />I will fix them and report back.