Hard, hot starting and dieseling, 67 big block

[Dave C.]

Just been working out the bugs in my 67 big block, it has a 1969 400 horse tri-power, installed at the moment, I had a stripped inlet on the front carb, so I pulled them all off and checked all the jetting, wich turned out to be perfect, replaced the float bowl and reinstalled everything, retimed it and it runs a lot better , but it is really hard to start when it is hot, I have tried every type of throttle application and you basically just have to keep cranking it, and pump the throttle like crazy to get it to start. Just wondered if anybody had any suggestions. Oh and also it diesels quite badly sometimes , on shutoff.

thanks,

Dave

[Edward B.]

Re: Hard, hot starting and dieseling, 67 big block

Check the timing - may be too far advanced.

[Mike T.]

Re: Hard, hot starting and dieseling, 67 big block

Dave - It really does sound like the timing is off. Can you fill us in on the details like initial timing and at what rpm and also, once you've got it running, what your vacuum reading is showing. And let us know if the cam is something other than a stock hydraulic for a 390/400 horse Bigblock.

[Duke W.]

Re: Hard, hot starting and dieseling, 67 big block

If you have a '69 center carburetor, it has ported vacuum advance. Convert it to full time, and you need a B26 12" VAC, assuming the cam is OE. If not, follow the Two-Inch Rule.

Check the VAC - numbers on the bracket? Does it work? One common cause of "dieseling" is insufficient total idle spark advance, and ported vacuum advance or a VAC that doesn't meet the Two-Inch Rule or a dead VAC with full time vacuum advance is a recipe for run-on when you shut it off.

While you're at it you should also record the centrifugal advance. Chances are your spark advance map is screwed up, especially if it has an aftermarket cam that doesn't pull as much idle vacuum as the OE cam.

As previously stated, you need to report normal idle vacuum/idle speed in neutral if a manual and Drive if an auto.

Duke

[Dave C.]

Re: Hard, hot starting and dieseling, 67 big block

I had a mechanic friend of mine helping me out with that stuff, I believe the timing was set to 32 degrees total, it had been running at 55 degrees when I got the car, I don't know about the vacuum , and have no idea what type of cam is in the car.

[Duke W.]

Re: Hard, hot starting and dieseling, 67 big block

There are three "total spark advance" numbers:

1. Total idle spark advance
2. Total cruise spark advance
3. Total WOT spark advance

Which one are you talking about?

Idle vacuum at specified idle speed is critical data to understand in order to set up an appropriate spark advance map, and it will also give us an idea whether the engine has an OE cam, or a higher overlap aftermarket cam. The OE spark advance map may not be appropriate for the latter.

We also need to know if the vacuum advance is ported or full time.

Without having some basic data about your engine, there's little specific advice that any of us can offer.

Whenever confronted with an operational problem such as yours, the first thing to test and document is the spark advance map and make appropriate changes if necessary. If that doesn't fix the problem the next most likely suspect is the fuel system, but in all cases a baseline is needed - how is the engine currently configured, and what is the basic idle behavior as expressed by manifold vacuum at the normal hot idle speed that you specify.

Note that hard hot starting is common with today's fuel, but the run-on problem is likely due to an inappropriate spark advance map.

Duke

[Timothy B.]

Re: Hard, hot starting and dieseling, 67 big block

Just been working out the bugs in my 67 big block, it has a 1969 400 horse tri-power, installed at the moment, I had a stripped inlet on the front carb, so I pulled them all off and checked all the jetting, wich turned out to be perfect, replaced the float bowl and reinstalled everything, retimed it and it runs a lot better , but it is really hard to start when it is hot, I have tried every type of throttle application and you basically just have to keep cranking it, and pump the throttle like crazy to get it to start. Just wondered if anybody had any suggestions. Oh and also it diesels quite badly sometimes , on shutoff.

thanks,

Dave

Dave,

The hot start issue could be because of fuel percolation, try the engine with the heat riser wired open and get more timing in at idle. Additional timing will enable you to close the throttle blades more and stop the dieseling after shut down.

[Dave C.]

Re: Hard, hot starting and dieseling, 67 big block

Had another mechanic friend , drop by this morning to have a look, he put a vacuum gauge on it and it seemed to be fine he reset the timing to 12 degrees initial and 38 degrees total, ( it has a unilite distributer with mechanical advance) he reset the float levels ( they were slightly high ) and things are getting better , but as timothy mentioned the front carb is percolating, and my friend thinks that maybe it has the wrong float bowl gasket , as he says there are many different varieties, with different holes blocked off in different locations, so he is going to look into that for me tomorrow. someone else mentioned that the later cars had a device that replaced the idle adjustment screw and at ignition off , shut the center carbs throttle plates , maybe I could retrofit one of these units?

[John H.]

Re: Hard, hot starting and dieseling, 67 big block

Had another mechanic friend , drop by this morning to have a look, he put a vacuum gauge on it and it seemed to be fine he reset the timing to 12 degrees initial and 38 degrees total, ( it has a unilite distributer with mechanical advance) he reset the float levels ( they were slightly high ) and things are getting better , but as timothy mentioned the front carb is percolating, and my friend thinks that maybe it has the wrong float bowl gasket , as he says there are many different varieties, with different holes blocked off in different locations, so he is going to look into that for me tomorrow. someone else mentioned that the later cars had a device that replaced the idle adjustment screw and at ignition off , shut the center carbs throttle plates , maybe I could retrofit one of these units?

Dave -

If you had the proper vacuum advance distributor, you could stop the dieseling instantly; having a centrifugal-only distributor will prevent you from closing down the throttle plates. The device you mentioned was the "anti-dieseling solenoid" which was a band-aid added when they went to "ported" vacuum, which allowed 100% closure of the throttle plates at idle to "stop the shaking". Without being able to increase idle spark advance, you can't close the throttle any further at idle.

[Timothy B.]

Re: Hard, hot starting and dieseling, 67 big block

Dave,

The full manifold vacuum advance is the best approach because the benefits of vacuum advance go far beyond the problem you are having, but if all you have is a mechanical advance distributor then you could try the following to see if it helps.

Set the initial timing at 18-20* and then limit the distributor centrifugal advance to 20*. If the engine will crank without kick back that may help cool the engine and speed the idle rpm which will enable you to close down the throttle blades more.

Don't forget that the centrifugal advance needs to be changed in the distributor so a over advanced situation does not damage the engine, you only want approx 36-40* total WOT timing (18* initial + 20* centrifugal).

[Duke W.]

Re: Hard, hot starting and dieseling, 67 big block

The best solution is installing a Delco single point tach drive distributor and setup a spark advance map that's suitable to the current engine configuration.

Duke

[Dave C.]

Re: Hard, hot starting and dieseling, 67 big block

Thanks guys, I like Tim's suggestion, and will give it a try, I would like to try and avoid swapping out too many parts, at this time, as I believe this car to be an original L-71, and I already have a date coded, broached, restamped long block for it, compete with the TI ignition setup. I would just like the L-68 to get me through the summer, while I collect some more parts.

[Larry M.]

Re: Hard, hot starting and dieseling, 67 big block

Thanks guys, I like Tim's suggestion, and will give it a try, I would like to try and avoid swapping out too many parts, at this time, as I believe this car to be an original L-71, and I already have a date coded, broached, restamped long block for it, compete with the TI ignition setup. I would just like the L-68 to get me through the summer, while I collect some more parts.

With today's pump gas (93 PON max) I would limit total advance to 36 degrees with the L-68 big block: 14 degrees initial and 22 degrees in the distributor. If you use higher octane gas, than you could push for the higher total advance.

Just my (experienced) opinion. FWIW.

Larry

[Dave C.]

Re: Hard, hot starting and dieseling, 67 big block

thanks Larry, I have been running Chevron 94, and I sure don't want to buy race gas for this thing, the way it goes through it, I don't remember my 66 L-72 going through this much fuel, ( not that I'm complaining , it is the best use of gasoline that exists )

[Domenic T.]

Re: Hard, hot starting and dieseling, 67 big block

Dave,
Take all the advice you can get here. I built engines since the 50's and was a was a chevy mechanic at a dealership and built the great engines ever since.
I had the same BB problem with a 425 HP 1969 that I installed in my chevelle. It ran great ( with normal tune) on a mix until I tried to use our 91 pump gas. I was mixing about a 50/50 mixture of av gas and went to pump gas after de tuning (retarding the timing) and it seemed to run OK but would diesel on shut off.
I got used to the power loss but it would diesel if I didn't stop it in gear with the A/C on (automatic trans).
I stopped it one day and it dieseled violently. Now it has piston slap on number 7 cylinder. Think I broke a piston skirt! Never had that happen before but never had such violent dieseling because I always mixed with av gas.
Didn't get the slap noise till it did that, all happened the same day over 1500 on new engine. I will post one day as to why a skirt broke and if there was a gremlin that the dieseling woke up.

DOMP

DOM