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Rebuilt 1275 slowly getting dialed in

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TulsaFred

TulsaFred

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So my cheap rebuild is doing pretty well, but still working out a few kinks.
Good news is good compression on all cylinders (155-165 each).

The engine smoothed out greatly after replacing the throttle shafts on my SU's.

However:
1. Still wants to idle at about 1200 rpm even with idale speed screws and fast idle speed screws backed out all the way.

2. Oil pressure is about 60 psi on start up gradually dropping to about 35-40 when fully warm at idle.

Thoughts?

Fred
 
Oil pressure sounds fine. You must still have an intake leak if you can not get the idle lower. I can drop the idle on mine to about 500 before it stalls out. And that is with a worn out cam.
 
Got rpms down to 1000 or so
Just cant find another vacuum leak
Clearly the throttle shafts were leaking
I notice if i push down hard on throttle linkage it will drop a hundred rpm or so, but thats probably just sealing the butterflys tight with the force

Thinking perhaps too much advance with the vacuum

Advance with vacuum plugged at ile about 10 degrees

Maybe 5 would be better
 
Are you sure that the throttle plates are closing correctly? Since you replaced the shafts, they might not be fitting perfectly. Even a very small gap is enough to do what youare describing.
 
You could be right Steve
Sounds plausible

Fred
 
Make sure they were installed the right way around. there is a bevel on them, so the fit only one way and should close all the way. Also check that your linkage initiates opening the throttle <span style="font-style: italic"><span style="font-weight: bold">precisely</span></span> at the same moment. This is a common mistake made when setting up duals.
 
Thanks Guys
I'm suspecting an issue with the throttle plate install.
I didn't mention, but my carbs had the poppet valves in them, and when I replaced the throttle shafts, I also replaced the throttle plates with a solid set (no poppet valves) from a spare old nasty set. The spares were very corroded and I had to use a fair amount of force with pliers to pull them out of the old throttle shafts.
Cleaned them up on my wire wheel and they looked like new since they are brass. However, I'm wondering if I could have inadvertently caused a slight bend or warp when removing.

I did look pretty carefully at the closure of the plates when reinstalling and did notice the bevel and made sure they went in correctly. However, if there was just a very slight bend or warp, maybe it was so slight that I didn't notice it.

One thing I did notice, when I tightened the throttle plate attaching screws the operation of the plate started to bind slightly. Just on one carb, not the other which still operated freely. I re-loosened and jiggled the binding plate and tightened again. That helped but there was still a slight bind. I couldn't see what was causing it and it seemed to close completely. Seemed to me the spring force would be more than enough to overcome the fairly slight binding.

However, something was a little amiss on that carb, and if, as Steve suggests, even a very slight lack of complete closure could cause my problems....

I'll be pulling the carbs again today and see about the fit of the suspect plate.

Have others had similar issues with throttle plate replacements?

Fred
 
When replacing throttle plates you need to align them with the bore. Set them up loosely with the screws just backed from snug and then wiggle the plate and shaft until all clearance (as much as possible) is eliminated. Shine light into the far end so you can see how much clearance you have between the bore and the plates. When you eliminate as much clearance around the plate edges as possible, carefully snug the screws and recheck. Then tighten the screws and recheck. Loosen and repeat as often as needed or as often as you can stand it. Repeat for Mr Other Carb. I had to do the same to mine when I replaced my shafts. I do it the same way for Holleys, Carters/Edelbrocks, Strombergs etc.
 
I meant to mention also to make sure that your jets are returned to the full stop (all the way up) position. Sometimes if the linkage is adjusted wrong (again, out of perfect sync) or the cable is a little to tight or the jest bind, one or both can be held slightly more down, making the mixture richer than it should be.

When you get your high idle, push up on each one and see if the idle drops.
 
It was the throttle plates. Putting the light behind them made it much easier to see the gaps.
I readjusted both plates and reinstalled the carbs and it now idles right down. I set it at 800 rpm.

It's purring like a kitten with good compression in all cylinders, good oil pressure, and steady temp of 180 degrees.

Not bad for a cheap rebuild (just a few hundred dollars all in). Plus, I bought the engine for $200 with a "free" midget attached...

The engine test stand was well worth it. Saved a lot of trouble working out the kinks on the stand.

Now I just need to pull the harmonic balancer/send for rebuild, and fix the seal behind it on the timing cover. Leaking there but not on the rear...

Fred
 
Fred,
Good job on the throttle shafts and discs. With new shafts and discs you should have an easy time of isolating any other fuel/spark problems that might pop up.
Take care, Rut
 
Thanks Rut.

A few thoughts for others contemplating a rebuild:

At the machine shop I had the block hot tanked, new cam bearings and valve guides installed, and had them mic everything, about $300 labor.
They advised a .030 over rebore, turn crank .010, hardened valve seats, deck block and head, new valves. That would have meant new pistons as well, of course.
The total would have been thousands of dollars.

I thought about it, and it just seemed overkill for a little fun project car for occasional use on the street. Plus, the wear on the cylinders, crank, etc. seemed minimal to me (though I'm no expert).

I was feeling that the machine shop, while probably technically correct for a professional rebuild, was incentivized to do as much work as possible. My incentive was to do the least. I decided to go "cheap" and figured I could always pull the engine and redo it if it smoked, ran poorly, etc.

So I took the block/crank/heads/etc. back home. I bought a 3 stone hone at Harbor Freight, and honed the cylinders nicely. I bought rings/bearings/valve springs/lifters from VB. Got a Payen head gasket. I reused my oil pump, valves, pistons/rods, pushrods, rockers, etc. Lapped the valves in, installed the rings and bearings,torqued everything to specs and reassembled.

So far I'm glad I went "cheap" as the engine seems to be doing great. Doesn't smoke at all and seems to run perfectly on the stand. We'll see how it goes once I get it on the road.

Hope this helps,

Fred
 
Fred:

I'd agree with what you did. I just took a low mileage 948 engine and did the same thing. It runs fine.
If all dimensions are pretty close and cylinders are not badly scored or scratched, you can do OK with a "sympathetic" rebuild, as long as you do not intend to run at high RPMs, add a supercharger or go racing.

With my race engines, it's a different story.
They need to be real good, so I'll always invest in a rebore and regrind on the crank.
But a lightly driven, stock street engine can be given more life by just reringing, appropriate new bearings and cleaning up the valves, etc.
 
Thanks Aeron,

High performance and racing would certainly demand a more comprehensive rebuild.

I would say, though, that even a mild street 1275 will likely be quite a performance boost over my mild street 948!

Fred
 
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