Hot Rod Forum : Hotrodders Bulletin Board - View Single Post - Running rough under load mystery.
View Single Post
  #15 (permalink)  
Old 09-20-2013, 09:54 PM
martyminnesma martyminnesma is offline
Registered User
Join Date: Sep 2013
Posts: 7
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts

It should be more stable than that, you may need a regulator in the line which will smooth the pulsations and allow a tighter adjustment on the pressure which should be 4 to 6 psi.

is a mechanical pump without a regulator normally like that? it goes berserk when i rev it up too. im guessing its not abnormal considering its a diaphragm driven pump.

You said it has a mild cam having the specs would help determine if the 15 inches reflects a wilder mild cam or whether the cam is retarded to the crank. What you're seeing is normal the off idle vacuum comes up with no load on the engine, dropped shut the vacuum will jump up then fall back. Here we are talking about the cam being retarded rather than ignition, but changing that should have an effect on idle speed and vacuum.

k cam spec, they dont mean much to me but here it goes:

cam lift intake .281
exhaust .296
valve lift intake .422
exhaust .444

SAE duration: intake 278 exhaust 288
SAE Timing: BTC 27
ABC 71
BBC 76
ATC 32

I hope that tells you something. I remember when putting on the cam and timing chain and everything that i set the cam at 4 degrees advance, they gave a little mark on the gear for it which told me which was what. I'm 100% confident i got that right. and i only have about 4000miles on it since the rebuild. is it possible for the chain to stretch that much to put it out of time?

funny thing too, if i let the manifold remain uncovered,

What does it mean the manifold uncovered?

I mean the T fitting behind the carb, I leave one side of it unplugged to open air, and the truck seemed to sound nicer when i revved it up. but it maybe idled worse.

A vacuum leak would drop idle manifold vaccum but certainly would not cause the engine to run rich unless it was so large as to pull the carbs power enrichment circuit on but that would be a lot lower than 15 inches at idle. Another way of "forcing" a rich mixture is a plugged air filter. I had earlier mentioned a plugged exhaust this keeps burnt gases in the cylinders which slows combustion making the engine appear to be running rich on the carb and retarded in timing. Take a look at the exahust system for crushed pipes, mufflers that collapsed internally, and look at the heat riser damper if it has one to see that it is open (or closed) this thing forces all the exhaust through the exhaust crossover of the intake when the weather is super cold, like what you get in the Great White North.

the exhaust seems to be in fine shape. and these heads don't have the port for the exhaust cross over. so that cant be it. i think it would run noticeably worse if the exhaust was blocked on one side. both pipes sound pretty even.

Propane back to gas OK this makes sense! OK base timing is set without vacuum advance, the centrifugal should be inactive at idle but become active about 1000-1200 RPM and should be all in by 2000 to 3000 depending on the spring choice and where the weights are physically stopped if such stops are on the plate they swing from. If there is no physical stop and the springs are preventing further advance then a softer spring should be used as with 10 degrees base there sould be about 26 to 30 degrees in the centrifugal. The vacuum advance is there to provide advance sensitve to load which affects manifold vacuum. This will be higher with low throttle opening which create high manifold vacuums and will decrease as the throttle is opened, the assumption here is the engine RPMs are going up so the centrifugal will take over. The problem is in mountains where the RPMs may be fairly low and the throttle quite open so the mainfold vacuum is low, this can be a hole in the need for advance, the assumption made by the designers is that in this situation the mixture density will be high which adds speed to the burn naturally. Speed of the burn is what these advance systems are all about. The burn is proportional to the compressed density of the mixture in the cylinder. At low RPMs with a closed or nearly closed throttle the density is low requireing a lot of advance. As the throttle is opened and RPMs rise the cylinder density increases as does the burn speed but under 3000 RPM it is not proportional so systems need to be applied to start the burn early enough for the best cylinder pressure to occur from 20 degrees after top center to about 65 degrees. From 3000 RPM up for some reason mother nature takes over and the burn speed increases on it's own from there on up the RPM band for quite a long time.

k that all makes good sense actually, had to read it about 5 times haha. yeah the weights are really not opening that much. they basically only add about 8 degrees. it doesnt feel sticky though, maybe i'll try a lighter spring and see what that does, but i would think the medium spring would open eventually.
also the HEI condenser tested fine, the coil tested as ok, but that doesnt mean much when its off and cold.

Is HEI original to this truck? If not, was it wired properly to feed it full battery voltage all the time?

yeah im pretty sure they got it right, its been running on hei for 30 years now, im pretty sure i checked when i rebuilt the motor, but i'll double check again.

If you put in a roller cam the original HEI gear if it is GM of that era would not be compatible with the cam's material. Usually the distributor gear wears in this case but sometimes it takes the distributor driving gear of the camshaft with it. Timing would be erratic and lean toward retarded if this is the case. I'd pull the distributor and take a look at its gear. Also how many miles are on the timing set between crankshaft and camshaft?

i had to look up roller vs flat tappet cam, I have a flat tappet cam, so i'm hoping that wont mess with the gears? i had the distributor out a few days ago and didn't notice anything odd about the teeth on the gear, but i'll double check.

thanks so much for all your help, this is helping me narrow it down big time. tomorrow I'm going to;
try lighter springs in the distributor to get a proper advance out of it.
check the spark plugs for a good hot spark
check the gears on the distributor
check voltage at the hei
possibly just replace the plugs and wires
possibly try a fuel pressure regulator.
Reply With Quote