Timing and Dieseling

[maxtwms]

Alright I'm pretty new to this and this might be my first post so here goes the issue. I have an 85 K10 with what I'm told was a "rebuilt" 305 (no clue why). It has a Holley 4175 650 cfm and 4 spd manual trans if that matters.. I went to set timing, idle and mixture so here was my approach. I left brake booster, pcv, and choke hooked up to manifold vacuum. Distributor advance port is open (don't think that matters). Teed the choke line to a vacuum gauge. Started the truck and went to establish initial timing to (assumed because idk if the harmonica balancer can be installed at wrong angle) 12deg advance. Couldn't get idle down and found choke was stuck. Flipped choke arm down between electrical unit on the side of carb and the carb body. Had better idle at this point. Found 12deg mech advance while feathering the curb idle and mixture screws. Now I moved curb idle to get 750 rpm and turned mixture screws in and out at same distance to find max vacuum. I felt like I noticed a thin white fog of smoke but I didn't smell gas and the idle seemed to be fine. All the number I thought to be correct had been met. Vacuum gauge was vibrating around 10in. Not great but best I could get. I got in the truck and could hardly get it to move. Almost like I had a vacuum leak. I manipulated hoses but no change. I finally got it back in the parking spot and cut it off when it dieseled for.like 15 seconds... I stepped back waiting for a disaster. This was the worst it's ever run on. I was trying to get a sewing machine. Wth is happening! What did I miss? Am I supposed to hook the vacuum advance distributor back up somewhere in between the process. Sorry for the rant but I'm getting worn down here. Thanks in advance.

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[maxtwms]

A few problems I suspect and maybe y'all can point me to the likely culprit.
I have no fuel filter and run ethenol free gas (just realized today idk the octane)
At some points in the tuning process (while learning) the mixture screws had little to no effect on engine. Even when turned completely in.
Have not replaced spark plugs. (Old timing light didn't flash consistently. Not sure who's to blame here)
Air filter was off for entire calibration.
Carb or manifold gaskets could be letting air in although if I had 20in of vacuum at any point, no matter the idle (around 1500 at most with prob 50deg advance no vacuum) could that still be true?
Thinking of a carb rebuild kit while I decide if I have carbon buildup on my heads from running rich all these years from PO setup.
Do I have to chase all these leads down?

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[Frankenchevy]

Did you test your brake booster for leaks? The harmonic balancer can break free from the rubber, then your timing marks won’t be accurate. You can find tdc by pulling #1 plug to see if it corresponds to the HB mark.

 

[maxtwms]

Did you test your brake booster for leaks? The harmonic balancer can break free from the rubber, then your timing marks won’t be accurate. You can find tdc by pulling #1 plug to see if it corresponds to the HB mark.

I haven't checked the brake booster since it was a solid tube. Prob neive for me to ignore but I'll check. What do you mean by balancer breaking free from rubber? I'm not entirely sure how they are mounted although id assume they are keyed in some way. By TDC Mark is a machined notch hilighted by paint. I plan to check TDC once I change spark plugs. As soon as I get a 1 and 2 yo asleep at the same time..

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[Frankenchevy]

I haven't checked the brake booster since it was a solid tube. Prob neive for me to ignore but I'll check. What do you mean by balancer breaking free from rubber? I'm not entirely sure how they are mounted although id assume they are keyed in some way. By TDC Mark is a machined notch hilighted by paint. I plan to check TDC once I change spark plugs. As soon as I get a 1 and 2 yo asleep at the same time..

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The brake booster itself can leak, not just the tube leading to it. The outer ring of the HB is rubber isolated. The rubber can degrade, allowing the outer ring with marks to rotate independent of the crank.

 

[maxtwms]

The brake booster itself can leak, not just the tube leading to it. The outer ring of the HB is rubber isolated. The rubber can degrade, allowing the outer ring with marks to rotate independent of the crank.

Man this would make so much sense if that's the case.... Bad timing would/could be the source of all of my problems. I'll suck it up and check once and for all my TDC. If nothing else I sound like I know cool things. Will report back. Thank you sir

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[HotRodPC]

The dieseling and run on is due timing being to far advanced. If all you have is 10hg vacuum, that is piss poor. If turning the AF mixture screws doesn't make a difference at all, that's a tell tale sign the carb is in disarray. That kinda goes along with bad vacuum. You probably have a dirty idle circuit or some clogged orfices in the carb. This of course is based on your explanation since I don't get to lay hands on it or hear it.

Have you found the thread where you use a vac gauge to a diagnose an engine and tuning problems???

 

[Frankenchevy]

Man this would make so much sense if that's the case.... Bad timing would/could be the source of all of my problems. I'll suck it up and check once and for all my TDC. If nothing else I sound like I know cool things. Will report back. Thank you sir

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No problem, I’m just here learning like you. Vacuum leaks and timing will drive you nuts, but are great learning experiences. Good luck!

 

[maxtwms]

The dieseling and run on is due timing being to far advanced. If all you have is 10hg vacuum, that is piss poor. If turning the AF mixture screws doesn't make a difference at all, that's a tell tale sign the carb is in disarray. That kinda goes along with bad vacuum. You probably have a dirty idle circuit or some clogged orfices in the carb. This of course is based on your explanation since I don't get to lay hands on it or hear it.

Have you found the thread where you use a vac gauge to a diagnose an engine and tuning problems???

Exactly my thoughts. I did see the page with the 13 scenarios. At first I had about 19inhg flickering which I thought was good (supposedly a diff cam which I chalked up to PO saying engine was rebuilt) and was excited but once I moved the timing more towards 12, the vacuum dropped and I was confused about everything. My TDC Mark must be off.. I can only imagine how confused I'd be if I hooked up the distributor advance to manifold with enough improvement to baffle me. .... Maybe

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[maxtwms]

The dieseling and run on is due timing being to far advanced. If all you have is 10hg vacuum, that is piss poor. If turning the AF mixture screws doesn't make a difference at all, that's a tell tale sign the carb is in disarray. That kinda goes along with bad vacuum. You probably have a dirty idle circuit or some clogged orfices in the carb. This of course is based on your explanation since I don't get to lay hands on it or hear it.

Have you found the thread where you use a vac gauge to a diagnose an engine and tuning problems???

Wait.... Too far advanced??? You mean retarded too close to TDC right? I'm def not too far advanced. I don't doubt that the idle circuit could be clogged though. Trying to get a 3-206qft kit here Thurs

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[HotRodPC]

Yes, advanced timing causes dieseling and run on.

 

[maxtwms]

Yes, advanced timing causes dieseling and run on.

Ok I'm not educated enough on the subject but before, I was dieseling just a slight bit every now and then. Then when I turned my distributor to bring the mechanical timing to 12deg btdc, the dieseling got worse. I could not imagine retarding my timing further. My truck might just disassemble itself!! Lol am misunderstanding the terms here?

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[1987 GMC Jimmy]

I think you've got the terms backwards. If you advanced the timing more, you'd get more run on (12* BTDC + X*). If you retarded it, it would subside (12* BTDC - X*). I've learned this much through experience. A 305 is not a 350 many ways, and one of them is timing. It will not run happy with that much initial timing. My 305's at seven or eight degrees BTDC, and because I haven't just gone out there to deak with that aspect of it, I have to run premium in it, or it'll diesel and ping. I'm thinking five is probably the max I can go with mine, and I know four is okay. 10" of vacuum as your best reading is real rough. You have to have a serious vacuum leak for it to be there, and I don't think all of the little stuff leaking added up could culminate to that low of a reading. A bad intake manifold gasket set could, as could a carb that has a very damaged base gasket, isn't properly bolted down, or if the carb itself is somehow damaged. I don't know how the latter could happen on a Holley (although I'm sure it could), but it could definitely happen on a Quadrajet. I wouldn't jump to conclusion just yet about your timing indications being wrong because of some problem. A 305 just can't take the initial timing a 350 can.

 

[Charlie]

Could possibly be worn/stretched timing chain that can make setting timing difficult. I had this on my 350 last November and installed timing chain kit. Solved my problem.

Just an FYI.

 

[hatzie]

If this was "rebuilt" by a previous owner lord knows what they actually did to it.

I'd do the #1 TDC sanity check to be sure a 40 year old balancer doesn't have a freewheeling outer ring.

How is the oil pressure on the "rebuilt" 305?
The reason I ask. My grandfathers' last car before the Alzheimers forced us to stop his driving was a 1980 Impala with a tiny 262?? V8 and lots of highway miles towing a trailer in the winter from Central NY to New Mexico and Arizona. It had a cam failure at over 210,000 miles. A shop replaced the cam without replacing the oil pump... instead of pulling the motor and installing a decent running used 350 or 305. I figured they took advantage of him. The lifters wouldn't pump up properly and it ran like it had a second cam failure. The bearings checked out OK on visual so I installed a high volume oil pump in it. I lucked out. It ran just fine with real oil pressure and better after some adjustments to the rockers. It never had any power to speak of before and it ran OK, after the ordeal, til we had to stop him driving.