bowtiescottsdale

So here is how it went down, just messin around with the 1983 C-10 tow truck with some friends and seeing what i could get out of it.

-The motor in the truck is a 350 290hp crate motor, and the trouble i kept having was it cutting out at about 4K rpm in second gear, it would come back on after i let out of it.

-The motor has the 305 intake manifold, rochester Q-Jet, stock GM HEI, nothing special.

The fuel pump is an 80 gph Holley mechanical pump.

Just wondering if there were any ideas out there?

ZONES89RS

iTrader: (25)

So first gear is fine? How about third?

bowtiescottsdale

well 1st automatically shifts at 4800 with my 700R4 but as it gets near 5K rpm in first, it does feel down on power, and when i go through the traps in 3rd gear it feels down on power in the top end. It almost felt like it was starting to miss. idk how many rpms i was turning through the traps but it was hardly pulling and falling on its face.

Some of my friends were saying it could be a bad vacuum advance because it is ok in the bottom end, but idk if that makes sence...

Just looking for suggestions.

ZONES89RS

iTrader: (25)

Well, it could be starving for fuel, check your timing. Your vacuum advance is only functional under part throttle, has nothing to do with WOT. So, make sure your timing is not way back, toy might want to get a electric fuel pump but what you have SHOULD be good enough. Male sure you have your carb adjusted correct if it is a holley, the floats might need to be set higher.

bowtiescottsdale

alright i will check timing... how much should i be looking at... about 32*? and the carb is a Rochester Q-Jet.

ZONES89RS

iTrader: (25)

Well, timing should be about that at total advance, so make sure you disconnect the vacuum advance and plug it off at the carb while tuning. I hate to say it but that carb might not be in tune ither.

bowtiescottsdale

i agree with carb. being a part of the problem also, but i can never get a straight answer ever, when the truck is at idle with the vacuum disconnected... how many degrees of timing should i set to when i throw the light on it.

bowtiescottsdale

bump... help me with timing this motor!!!!!!!!

blk/slvr02ss

iTrader: (21)

I would 36 degrees of total timing.

bowtiescottsdale

so your saying, when it is idling... with the motor warm... put a timing light on it and set to 36 degrees?

Sorry for my ignorance.

hookemdevils22

iTrader: (12)

the vacuum advance won't affect much at WOT. it's for part-throttle driving.

here's a great write-up to set your timing.

It would be a good idea to get a long piece of tubing and hook up a vacuum meter to your carb. drive it around and note your vacuum at various speeds and loads. this will help with tuning.

by chance, were you in the truck with giant rear meats out at firebird last friday?

bowtiescottsdale

thanks for the help man, and no it wasnt me, i live in NC.

so... im still looking for an answer... with my VA disconnected at idle... what # should i shoot for on the balancer?!

pancherj

I almost NEVER post on forums, but I thought I would try to help you out with some general timing info. Your base timing (what you will read on the balancer with the engine warm, vacuum advance disconnect and idle) is controlled by the position of the distributer only (unless your distributer is worn...that's a different topic!). Generally speaking, you should set it in the neighborhood of 8 to 12 degrees advance. Your total timing is the amount that is built into the system as the RPMs go up and the distributer internals turn faster. Now, the total mechanical advance can vary. If you have a smog era distributo, you might only get 20 degrees of mechanical advance. That 20 degrees plus the base timing (say 12 degrees) would give you 32 total. The advance curve determines how fast this extra mechanical advance comes in. With a performance motor, you want it all in by 3000rpm (give or take...all engines are different). If you are measuring the mechanical advance with an adjustable timing light, make sure the vacuuum canister is disconnected. Lastly, the vacuum advance is controlled by the engine vacuum built up at part throttle. It can be quite a lot in some circumstances. I have my engine puttin in 45 degrees total at some points. Again, each engine wants what it wants. Hope this helps!

blk/slvr02ss

iTrader: (21)

I never use the vac.advance on the older SBC i just use a dial timing light and set it to the total timing i want and zero it on the timing tab.

pancherj

Locking out the vacuum advance is another approach, but I wouldn't recommend it on something you plan on driving a lot (unless you don't mind a drop in gas mileage). I did this on the 406 in my old 1975 Nova. When I switched back to a dedicated vacuum advance distributer, it was the difference between 8MPG and 12MPG. Of course, if you have high compression and are up against it with pump gas, you may want to stay away from the vacuum advance (to be safe) and live with the poor MPG.

There is also the concept of totally locking out all advance. This is a racecar only procedure that has been carried over to the street. I know guys that lock their distributer at 35 degrees advance. This means it is ALWAYS at 35 degrees advance (idle...WOT...etc). It makes for a hard starting car and will also do nothing good for MPG. On a racecar, you really don't care because it is always at WOT.

bowtiescottsdale

thank you fine sir! I suggest you post more often, your knowledge is useful!

pancherj

Glad I could help. I was reading back through your posts. You said the engine was feeling down on power at around 5000rpm. It could be valve float. If the valve springs aren't up to snuff (or they were installed wrong), they could be going into float...it actually feels like the engine is missing and can be easily mistaken as timing or carb problems.

Another poster suggested getting a vacuum gage and hooking it to the motor. I couldn't agree more. It is the poor mans oxygen sensor!!

bowtiescottsdale

See i was thinking the same thing but i was always told it probly not... it is a crate motor, a cheapy 290hp 350, what would i see on the vacuum guage when the valves begin to float?

pancherj

bowtiescottsdale,

Good question. I have never tried to diagnose valve float with a vacuum gage (i use them more for setting up timing and carb adjustments). However, it would make sense that the gage needle would start to flutter when the engine goes into float. At WOT you should be seeing little to no vacuum (if you are hooked to the port that your vacuum canister is hooked to). If at 5000rpm (under load...try doing this test in low gear going up a hill) the needle starts bouncing around, I don't think that could be traced to the carb. Maybe timing, but not the carb. The timing you can check in park with an adjustable timing light. I doubt the engine will go into float with no load on the engine.

I don't know anything about the GM 290HP motor. You could probably get some info from the SDPC2000.com website (or other GM dealer websites). My guess is that it has a very mild cam and small springs (doesn't take much to make 290hp with a 350). The power band may only stretch to 5500rpm. Let me know what you find out.

Drumer919

iTrader: (2)

The 290 HP 350 is almost the same as a LO5 truck 350 with 76CC heads. The cam is a little different though.