1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

[Vincent F.]

Re: 1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

any recomendations on a new feeler gage?

[Duke W.]

Re: 1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

I have a large Craftmen set that goes from .0015" to about .035" in .001" increments that I've had for over 30 years. I've measured a few with a mic and they are close enough for valve adjustments.

Duke

[Vincent F.]

Re: 1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

Thanks Duke, this engine seems extremely noisy even for solid lifters which I havn't dealt with in years, and has a lack of power. Figured I would start here with fine tuning this beast. Distributor/points tuning and timing will be next.

[Michael H.]

Re: 1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

hi again,

everything started with the discovery that i had the wrong spark plugs on the engine. i had run 1600miles with the wrong plugs. as was recommended i changed to ac r45 instead of puting in the oe r43.

then i set the valve lash cold as per duke/john docs in .016" and exh .023"

If the problem started when you replaced the spark plugs and adjusted the valves, I would go backwards and see which caused the problem. It does sound like the wires/firing order may be incorrect.

[Wolf S.]

Re: 1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

If the problem started when you replaced the spark plugs and adjusted the valves, I would go backwards and see which caused the problem. It does sound like the wires/firing order may be incorrect.

the situatiion was a combination of a number of problems.

valve lash way too loose
carb dirty
fuel hose to carb slightly blocked
carb base to intake vacuum leak
wrong spark plugs
centrifugal advance was swimming in heavy grease
and most of all
no distributor timing advance
accelerator cable binding

the engine runs fine now. hopefully just the fine tuning is left to be done

[Wolf S.]

[Duke W.]

Re: 1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

From the limited spark advance map data you provided, it looks like the centrifugal curve is the lazy OE, emission control version.

According to Terry's data you would have to rev the engine close to 6000 to achive maximum centrifugal.

Get a spring kit (use the OE weights) to get the centrifugal in as quickly as possible - say about 2500-3000. Then you don't have to rev it close to the redline to see what the max centrifugal is.

The 98 RON gasoline available in Europe is only a point or two less than sixties USA premiums, so you should be able to run a pretty aggressive curve.

Duke

[Wolf S.]

Re: 1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

hi duke,
thanks again for the input.

i'll try to get more advance data next week.

as to tackling the advance curve, i have two sets of springs--one is from mr gasket and the other cal custom (probably also mr gasket).
the trouble is which ones to use.

silver = heavy
grey = medium
gold = light

do you have a suggestion with which ones i should start with?
if i have the lazy oe curve (=heavy spring?), i should try out the lighter ones.

should i just change one spring at a time?

what about the advance limiter bush?

i suppose this is a trial and error thing.

[Duke W.]

Re: 1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

Thanks Duke, this engine seems extremely noisy even for solid lifters which I havn't dealt with in years, and has a lack of power. Figured I would start here with fine tuning this beast. Distributor/points tuning and timing will be next.

With the valve clearances set using the methodology and recommended setting clearances in the Hinckley/Williams paper, there should be very little valve train noise. If fact, you could probably fool most observers into believing it has hydraulic lifters.

Valve train noise is the sound of shockloading as the clearance is taken up at greater than clearance ramp velocity because the clearance is too loose.

Another problem that frequently arises is that the installed cam is sometimes not what the owner thinks. It's virtually impossible to positively ID a cam without removing is, but if you check gross lobe lift with a dial indicator, and it's within a few thou of the OE spec, that's a very good indicator that you do have an OE cam.

Duke

[Duke W.]

Re: 1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

hi duke,
thanks again for the input.

i'll try to get more advance data next week.

as to tackling the advance curve, i have two sets of springs--one is from mr gasket and the other cal custom (probably also mr gasket).
the trouble is which ones to use.

silver = heavy
grey = medium
gold = light

do you have a suggestion with which ones i should start with?
if i have the lazy oe curve (=heavy spring?), i should try out the lighter ones.

should i just change one spring at a time?

what about the advance limiter bush?

i suppose this is a trial and error thing.

Start with the weakest springs from each kit and determine where they bring in maximum advance. Use the quickest and set your initial timing to achieve 38 total. If the total is still 28 as per the OE specs, you initial should be 10.

Check the bushing on your distributor. If OE it's probably hard rubber or plastic. If it's in good shape don't change it.

If the engine doesn't detonate, you've got it about as good as you can. If it does detonate try one next heavier spring in combination with a light spring, then retest. You don't have to use the same spring on both sides, but by incrementally increasing one spring at a time to slow the curve, you have five different curves to choose from and a sixth curve by using one lightest and one heaviest.

It's always a matter of some experimentation with each individual engine, but the time invested pays excellent performance and fuel economy dividends.

Duke

[Edward C.]

Re: 1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

Dumb question, what is a B28 VAC? Ed

[John H.]

Re: 1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

Dumb question, what is a B28 VAC? Ed

Ed -

That's a VC-1810 vacuum advance unit, that starts pulling at 4" Hg. and is fully-deployed at 8" Hg.; they're stamped "B28" on the bracket.

[Edward C.]

Re: 1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

Thanks John. Ed

[Duke W.]

Re: 1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

VAC stands for Vacuum Advance Control, which it the official Chevrolet Engineering name for that silver thing on the distributor.

For new TDB participants take a look at this page and keep it in mind for future reference.

https://www.ncrs.org/lingo.html

The VC1810 is the NAPA part number for the B28, which duplicates the specs of the 1116236 OE Delco VAC. It's available in several other brands with part numbers unique to the brand, but all are stamped B28 and the same part made by Standard Motor Products.

You can find out more about VACs in the archives. One of only three of a dozen or more available are suitable for vintage Corvette engines through '74, either with full time vacuum advance or converted from ported vacuum advance. The other two are B20 and B22, and the archives explain how to pick the correct one for your specifie engine/trans combination.

Duke

[Jerry G.]

Re: 1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

Duke have your ever locked out the advance completly on a race motor? seems like this would take one more variable out of the race motor variables.I've heard that for racing motors the vacuum advance arm was welded solid to fix the plate and then the advance weights were locked down so you start with all advance in. It does eliminate the of over advancing at high RPM.