Strange stumble with BBC

« **on:** July 06, 2011, 06:53:10 PM »

So my bbc has a strange stumble that I'm having issues diagnosing. Base timing is set at 14 and total advance is 35 The stumble is present when I first take off and again just as the secondaries open. The carb is a Holley 750 and I first started out with 74/80 jets and a #55 power valve. Now I'm at 77/81 jets and a #65 power valve. The problem seems to be there but it"s only abut 75% better. I pulled some plus today and it looks like the motor might be running a bit on the lean side. The plug color is like a light gray and nowhere near the white when I first started running the boat. The ignition system is all new and fuel pressure is within specs. Any ideas? I was thinking of maybe going down to a #55 power valve and increasing the primary jets by one. Also, the timing at 14 has me kinda nervous since factory spec calls for a base of 10 If I bring down from 14 to 12 my total advance ends up at 33 which is h=good but then the stumble off idle is worse.

« **Reply #1 on:** July 06, 2011, 07:04:08 PM »

Quote from: N2GLOCK on July 06, 2011, 06:53:10 PM

So my bbc has a strange stumble that I'm having issues diagnosing. Base timing is set at 14 and total advance is 35 The stumble is present when I first take off and again just as the secondaries open. The carb is a Holley 750 and I first started out with 74/80 jets and a #55 power valve. Now I'm at 77/81 jets and a #65 power valve. The problem seems to be there but it"s only abut 75% better. I pulled some plus today and it looks like the motor might be running a bit on the lean side. The plug color is like a light gray and nowhere near the white when I first started running the boat. The ignition system is all new and fuel pressure is within specs. Any ideas? I was thinking of maybe going down to a #55 power valve and increasing the primary jets by one. Also, the timing at 14 has me kinda nervous since factory spec calls for a base of 10 If I bring down from 14 to 12 my total advance ends up at 33 which is h=good but then the stumble off idle is worse.

Exactly what I was thinking. Timing sounds a bit "off" to me. I think it might be time to really look into whats going on in your dizzy. Not saying just replace it, but I'd dig a little deeper into it and see if theres an appparent problem there. Points diz or electronic?

Ray

« **Reply #2 on:** July 06, 2011, 07:18:33 PM »

Quote from: IRRebel on July 06, 2011, 07:04:08 PM

Exactly what I was thinking. Timing sounds a bit "off" to me. I think it might be time to really look into whats going on in your dizzy. Not saying just replace it, but I'd dig a little deeper into it and see if theres an appparent problem there. Points diz or electronic?

Ray

Electronic ignition w/ an external coil. When I first got the boat, the distributor was rusted inside pretty bad. I tore it down and replaced all the guts. It has a new p/u coil, cap, rotor, advance stop bushing and springs. When I installed the new p/u coil I followed MSD's directions to a "T" and I know for sure that the gap is set correctly. I have the advance set up to where it kicks in pretty early. If I remember correctly, I installed the softest spring that were in the kit.

« **Reply #3 on:** July 06, 2011, 08:30:34 PM »

not that its the plugs, but what kind of plugs do you use?

i had my timing locked at 36* and carb primaries at 78 and secondaries at 82 on my 750dp. i used autolite 26's and never had an issue. dizzy's will create problems. try another dizzy and see if thats your problem. i had a burnt cap/rotor once, but it still ran good and had the rpm's are 5600.

whats been change to your engine?

« **Reply #4 on:** July 06, 2011, 08:54:20 PM »

Quote from: N2GLOCK on July 06, 2011, 06:53:10 PM

So my bbc has a strange stumble that I'm having issues diagnosing. Base timing is set at 14 and total advance is 35 The stumble is present when I first take off and again just as the secondaries open. The carb is a Holley 750 and I first started out with 74/80 jets and a #55 power valve. Now I'm at 77/81 jets and a #65 power valve. The problem seems to be there but it"s only abut 75% better. I pulled some plus today and it looks like the motor might be running a bit on the lean side. The plug color is like a light gray and nowhere near the white when I first started running the boat. The ignition system is all new and fuel pressure is within specs. Any ideas? I was thinking of maybe going down to a #55 power valve and increasing the primary jets by one. Also, the timing at 14 has me kinda nervous since factory spec calls for a base of 10 If I bring down from 14 to 12 my total advance ends up at 33 which is h=good but then the stumble off idle is worse.

Sounds like wrong pv to me....anything new(beside the ignition system)...when did it start??

« **Reply #5 on:** July 06, 2011, 08:57:52 PM »

Quote from: YumaJet on July 06, 2011, 08:30:34 PM

not that its the plugs, but what kind of plugs do you use?

i had my timing locked at 36* and carb primaries at 78 and secondaries at 82 on my 750dp. i used autolite 26's and never had an issue. dizzy's will create problems. try another dizzy and see if thats your problem. i had a burnt cap/rotor once, but it still ran good and had the rpm's are 5600.

whats been change to your engine?

I can try 78/82 jets as opposed to the 77/81 that I have now but our motors might be set up completely different. My friends Hallett has a bbc and a way hotter cam than me and his motor likes 74/80 in his 750DP It just seems kinda odd that my engine is requiring so much more fuel when I have a smaller cam. I have a pretty agressive intake and my choke horn is shaved off. Could it be that the combination of those two things is causing the motor to be lean since it's sucking in quite a bit of air?

« **Reply #6 on:** July 06, 2011, 09:11:59 PM »

Quote from: SLCKREG on July 06, 2011, 08:54:20 PM

Sounds like wrong pv to me....anything new(beside the ignition system)...when did it start??

I never really ran the boat w/ the setup it had when I bought it. When I bought the boat it had a Demon carb and Edelbrock mech fuel pump that were full of water. I switched to the Holley and I installed a new fuel pump. The intake is the same and I believe it's a Victor Jr. Besides that, all the other parts I replaced are just new parts to replace the old. Spark plugs are R44T which is what I pulled out of it. After I ran the boat a few hours with the 74/80 jets and 55 power valve the plugs were white. On this trip the engine stumbled bad from the fuel starvation. Things get better as I increase jet size. Now I'm at 77/81 and although things are better my problem is still not fixed. Just seems odd to me that this motor seems to like to be at 14 degrees base and still seems to be a little lean w/ the current jets.

« **Reply #7 on:** July 06, 2011, 09:52:34 PM »

I responded to your post on pb. Check to see that the accelerator pumps are not delayed.

« **Reply #8 on:** July 06, 2011, 09:55:32 PM »

You need to do two things. First verify the timing mark is at true 0* TDC. I have seen slipped balancers and timing tabs be off as much as 15*, also could be a broken woodruff key in the crank hub...

Another thing to do, this will sound silly is to go to a jet size you know will be fat, like an 80/84, depending on what your carburetor is. My 850 double pumper uses 80/80 jets and is only a teeny bit rich on the bottom and cleans out pretty good on the top......You are trying to tune it backwards from what we normally do, I go to where I know it will be rich, then lean it out. It takes less time IMHO...

Do those two things and also make sure the plug gap is like .044", she should run like a top. Betting the timing mark is off and you are running several degrees retarded along with a lean mix, together these are tough to narrow down...Had you started with a rich condition you could get away with a bit more timing and the problem would have been more apparent.

Good luck my man...

GT

« **Reply #9 on:** July 07, 2011, 08:08:06 AM »

Quote from: N2GLOCK on July 06, 2011, 08:57:52 PM

I can try 78/82 jets as opposed to the 77/81 that I have now but our motors might be set up completely different. My friends Hallett has a bbc and a way hotter cam than me and his motor likes 74/80 in his 750DP It just seems kinda odd that my engine is requiring so much more fuel when I have a smaller cam. I have a pretty agressive intake and my choke horn is shaved off. Could it be that the combination of those two things is causing the motor to be lean since it's sucking in quite a bit of air?

i had a mild 282 crane cam with a pretty much stock 454... at one point in the build.

« **Reply #10 on:** July 07, 2011, 12:24:24 PM »

another thing you can do is put your boat on the water and check how much vacuum your motor is pulling, you divide that number in half, and that is the powervalve you should be running...i dont recall what the factory power valve size is, but my old motor i did this exact thing to and at idle i had 15 whatever you call it for vacuum and so i went and got a 7.5 and wallah! also this was on a single carb tunnel ram and the plugs looked like it was fuel injected

also what pat said too...make sure that the springy dealios are just touching the accelerator pump arms...

« **Reply #11 on:** July 07, 2011, 01:38:27 PM »

Today I gave the carb a thorough once over and everything looks good. I had one of my techs here at work check to see if the squirters were working at the proper time and he said that it looked normal. When I changed the power valve, I gave the local carb shop my vacuum readings at idle and 2500 and according to him he gave me the correct power valve. I posted this problem on another forum and a couple of members mentioned that I'm probably having issues due to my current cam/intake combo. When I first bought the boat, I had a sneaking suspicion that the intake was probably gonna be an overkill. I'm fairly confident that I have the right advance spring/stop bushing combination in the dizzy so I'll leave that alone at this time. Either today or tomorrow I'll increase the primary jet size by one and I may switch out the #65 pv to a #55 again. At this point, I probably shouldn't increase the base timing since my total advance is already on the high side of the spectrum.

N2GLOCK

Strange stumble with BBC

IRRebel

Re: Strange stumble with BBC

N2GLOCK

Re: Strange stumble with BBC

YumaJet

Re: Strange stumble with BBC

SLCKREG

Re: Strange stumble with BBC

N2GLOCK

Re: Strange stumble with BBC

N2GLOCK

Re: Strange stumble with BBC

lbhsbz

Strange stumble with BBC

GT Jets

Re: Strange stumble with BBC

YumaJet

Re: Strange stumble with BBC

Nordie

Re: Strange stumble with BBC

N2GLOCK

Re: Strange stumble with BBC