mopower

Here's the question , 540 cu in , 1050 Dominator no power valves. What size jets??? Had 90's around , ran ok(625 hp/5300rpm) , decent exhaust temps. Went to 88's and temps came down a bit. (gained 7 hp too , 632 hp)
When you back off the throttle it pops out the exhaust and the plugs are pretty black(rich). To me , the way I see it is , it's still rich and dumping fuel into the exhaust , hence the popping when it ignites in the exhaust. Talked to Holley and they say go to 92 or 93 main jets
What'cha runnin???

mopower

Feel free to jump in here anytime

alcor

What are your primary main jets? I have to 500hp 454's with Holley 750 dual pumpers. When I bought the boat the plugs were black and it popped through the carb occasionally at higher rpm's (say 3800 rpm and up). Long story made short, finally determined that the engines were run out, had them rebuilt. After getting everything back in, still had the same pop. Pulled the plugs and they were now all white. Had the carb rebuilt by the best in our area. Still popped. Also noticed that after about 3/4 throttles, didn't really have any change in power. Checked the main jets and found 70's in the primaries and 80's in the secondary just like Holley said it should have. Believing it was running to lean on the top end and being the shade tree mechanic that I am, decided to start stepping the secondaries up 2 steps at a time, each time the pop occurred at a little higher rpm's and the plugs got progressively "duller" white. Plugs continued turning darker with each jet change. Now (with 90's as secondaries), plugs are light brown and no pop at top rpm of 5000 rpm. Also, now I have continuous throttles from 3/4 to full throttle.

So what the hell am I saying? I believe someone had probably changed the jets (leaning them out) to keep the dam thing running when I bought it. Are you the original owner? Have you asked Holley what the standard jets sizes were for that carb, primaries and secondaries? Perhaps your primaries are to rich and when you back off the sticks it's still pumping to much gas (thorough the primary main jets) causing the pop in the exhaust. Just a though, especially since no one else has "jumped in" on your original post. Let me know.

Al

mopower

Al, thanks for your reply. A 454 with a 750 and 540s with dominators are two different animals. There is a big difference in jets with or without power valves too.
I am the original owner of the carbs and engines. We'll get this straightened out. I was just wondering if anyone else had this set up and knew the jets they had

mcollinstn

A plug reading at your two critical throttle settings will be necessary to get it worked out. (set your 4 corner idle mix first, then verify float settings front and back - I know you already know this but a lot of tuning has been for naught cause somebody didn't verify these before attemting to jet). If you are NA then your air bleeds and such should be close enough.

#1) Figure out where your secondaries begin to open. Put a stop of some sort on the cable or somewhere so that you CANNOT get into the secondaries. Go out at full operating temp and do a plug run (fresh clean plugs, run at the throttle stop setting for 20 seconds, then pull the kill switches - yes, you CAN just back down, but I prefer to kill the ignition and pull the sticks all the way back - just my preference as idling can influence the reading). Jet for light tan or slightly leaner, depending on whether you are looking for best economy at high primary cruise.

NOW you got your primaries where you want them (you may even need to stagger the sizes in each barrel if you want to get picky).

#2) Do the same thing with WOT, 20 seconds if possible, or at least 10 full seconds. Kill the spark and check em. Jet again for light tan.

mopower

mcollinstn
The engine is on a dyno right now and have done just about like you said. After a pull , we try to kill it and the sucker becomes a diesel and continues to run...even under load. We can stop it but it takes quite a load to accomplish. Plugs come out relitively black , and with the exhaust popping on decelleration it certainly seems rich , yet a Holley is saying use bigger jets. we'll try this again since it only takes a few minutes to change jets and richer shouldn't hurt anything like lean can.

mcollinstn

mo,

If it is trying to diesel, I would be concerned about one of two things:

1) Wrong heat range spark plugs (causing the electrode to glow and continue undesired ignition).

2) Exhaust seat too narrow, giving the valve nowhere to dissipate heat (causing the same issue).

Neither of the above issues will show up on EGT.

Good luck in troubleshooting her.

mopower

mc,
You're right , but , these engines where run all last season (all 15 hrs with the sucko summer we had) , and did not have the popping problem. Carbs where messed with last summer in an attempt to solve or at least put a band-aid on and idle problem.
Did have Autolite AR133 plugs last year now have AR132 (colder).
Original prob. was no idle. Cams were too big and had virtually no vacuum at idle ( 3 to 4 inches) and when put in gear wanted to stall , plus peak HP was in the 6500 range (too high). This was due to Comp. Cams recommendation New cams were degreed in and are doing 625 hp @ 5200 and have 13 inches of vacuum. Put a load on the engine at idle and produced almost 100 ft lbs of torque and only dropped 200 rpms so the idle problem is solved.
Still believe the problem is carb related , but could be wrong .
We'll get it

mcollinstn

Regardless of how they ran last season, a motor that diesels has isolated hotspots in the combustion chamber that shouldn't be there.

It might only be a couple of cylinders doing it, but you got something glowing in there.

38KV

Ok,here is a far out theory.

You didn't tell if you pulled the heads for inspection before the dyno test.

If not,there is a chance it is plenty of soot in the chambers and on the pistons after
15 too rich hours,and the glowing soot now causes the diesel behaving.