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Old 10-19-2020, 11:21 PM   #1
Rich72C10
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Hard Starting Hot & Intermittent missing / hesitation

350 SBC rebuilt to stock with a "mild cam" and has about 300~400 miles on it. QJet 4BB (matching number for 72 from uscarburetors). It has a Top Street Performance HEI (has a stock look to it, with external coil).

When starting up in the morning or when has sit a rather long time, turn key and boom starts right up.

Drive around a good 5~10 miles to get good and hot/warmed up. Say I stop at the gas station, fill up, go into store to get a drink. I have to put the gas pedal to the floor to get it to startup and after which a bit of rough idle then clears up.

Timing was rechecked today, well because it ended up not being locked down! Also the fuel pump was replaced today (only part that was reused from before rebuild). Set to 12 degrees and has just shy of 16 for vacuum.
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Old 10-20-2020, 12:02 AM   #2
geezer#99
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Re: Hard Starting Hot

Could be fuel percolating causing a little flooding.
Did you touch the gas pedal?
Some motors don’t like you to even think about touching the pedal.
Or the new pump is putting out too much pressure.
Look in the carb when you shut it down hot. Look for any raw fuel.
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Old 10-20-2020, 12:18 AM   #3
Rich72C10
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Re: Hard Starting Hot

when I don't touch the pedal I am sure it will not start, crank and crank until I so hit the pedal.

I had to google percolating. My brother actually suggested a wooden clothespin. I thought he was pulling my leg. Non ethanol gas and most of the other things on Mike's Carburetor Parts percolating, I am not sure about being able to do.

I am not sure its the pump, since it did this with the old pump too. I'll check for raw fuel to get that off (or on) the list.

Thanks.
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Old 10-20-2020, 01:12 AM   #4
pjmoreland
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Re: Hard Starting Hot

My truck had that problem. I installed an Edelbrock Heat Insulator Gasket and now it starts right up after sitting a little while when hot.
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Old 10-20-2020, 06:24 AM   #5
geezer#99
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Re: Hard Starting Hot

Clothespin only works on Studes! And some Fords!
Quadrajets normally don’t have fuel percolation problems.
If you have a hot slot intake and the wrong base gasket they might.
What choke you got?
Post a pic of your carb.
Could see something amiss.

Info here on the ‘infamous’ hot slot intake.


https://www.corvetteforum.com/forums...d-problem.html
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Old 10-20-2020, 06:28 AM   #6
Rich72C10
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Re: Hard Starting Hot

But I have a QJet, are those generic to any carburetor?

Getting photos.
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Old 10-20-2020, 06:50 AM   #7
geezer#99
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Re: Hard Starting Hot

If you’re talking heat spacer every carb is different.
Quadrajet doesn’t need one. Normally. Hot slot intake does.
Eddy carbs do. Usually made from aluminum, phenolic resin (think plastic) or wood.
Holley uses a shield that deflects heat from the bowls.
The position of the float bowl determines the device needed.
Quadrajet bowl in a low heat zone above the plenum.
Eddy has two bowls. Each out on the high heat sides.
Holley bowls out front and back.

Another consideration on your starting problem is your choke setting. Too rich can act like yours.
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Old 10-20-2020, 07:00 AM   #8
Rich72C10
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Re: Hard Starting Hot

Choke, interesting. I think my brother talked about some choke problems or something. I poke him about that. He'll be up in about a hour.

I have photos but the form is giving me some error (security token was missing.). I'll keep at it.

Edit: uploaded forgot to resize them - doh.

Dead cold from overnight, gas pumped once. I am thinking I should have one after running it 10 miles or so and then stopped ... but it is just now 6am LOL
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Old 10-20-2020, 07:05 AM   #9
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Re: Hard Starting Hot

Make your pics smaller.
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Old 10-20-2020, 07:26 AM   #10
Rich72C10
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Re: Hard Starting Hot

For the record, I do not have that “hot slot” aka smiley face intake. I have the one exactly for my 72 SBC - got a NOS from North West (350ci V8 Intake Manifold 6268751). My Carb is matching for 72 SBC C10 Truck, exactly (7042208 1972 CHEVY GMC TRUCK).

Edit: While the sticker on the intake is not 7042208, it's a crossable number back to mine (this intake part number is for a Chevy/GMC boat that was made past '72). I had to get a new one because my stock one had a crack in it
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Old 10-20-2020, 07:27 AM   #11
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Re: Hard Starting Hot

Is this choke plate is opened up and sitting vertical when you try to start it again after it is warmed up?
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Old 10-20-2020, 07:39 AM   #12
Rich72C10
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Re: Hard Starting Hot

as soon as it's light, I'll give a good 10 mile drive and get a quick photo of the carb. Currently, I do not know. I am learning (and my brother says Qjets are not like carbs on a 30 Ford LOL - so he is learning too).
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Old 10-20-2020, 08:02 AM   #13
Rich72C10
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Re: Hard Starting Hot

I hate to mix things in but kind of on topic I think. With my mild RV Cam we are set to straight up stock 12 degrees. What is recommended for a RV Cam (generally), we are thinking to bumping it up to 15 and perhaps that will bring up vacuum closer to 17 or a bit more?
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Old 10-20-2020, 08:06 AM   #14
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Re: Hard Starting Hot

I see something I hope your brother didn’t do on your carb.
We can blame him anyway! LOL!
That nut you’ve got holding the air cleaner rod in place can hang up on the secondary rods.
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Old 10-20-2020, 08:09 AM   #15
Rich72C10
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Re: Hard Starting Hot

How can that nut hang anything but brother said - easy take it off LOL
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Old 10-20-2020, 08:13 AM   #16
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Re: Hard Starting Hot

Poke the secondary door wide open and watch the rod hanger. It might hit that nut.
Rod hanger goes straight up towards it.
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Old 10-20-2020, 08:46 AM   #17
Rich72C10
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Re: Hard Starting Hot

It did tap the leavers / hangers. Just a bit. Removed.

That would’ve caused my hot starting problem, right? Though I imagine it would cause problems with the hanger getting worn being banged up against that nut. Best I could tell it just slightly wouldn’t allow the secondaries to open 100%, hardly noticeable.
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Old 10-20-2020, 10:40 AM   #18
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Re: Hard Starting Hot

Look down the carb when it is running. Make sure you don't have fuel dribbling out of the primary boosters. This can cause a rich condition and when it's hot and you shut it off it essentially acts like an engine that has been flooded, hence having to open the butterflies open with your foot tp start.

This can happen because of various circumstances, not all that are obvious. Float level and non functioning needle and seat are an example but can also happen if the primary butterflies are open too much with the idle screw (improper tuning) which leads to too much carb signal.

I've worked on a handful of engines that have had this issue. Let me know what you find! If it is this I can tell you things to check and ways to fix it
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Old 10-20-2020, 10:54 AM   #19
Rich72C10
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Re: Hard Starting Hot

Some photos hot
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Old 10-20-2020, 11:42 AM   #20
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Re: Hard Starting Hot

When I wrenched for far too many years we would get cars/trucks in with QJ carbs that would have that issue. What I found a lot was the lead plugs on the bottom of the carb for the main jet passages leaked gas and then flooded the
engine. Which caused a hard start to the point you have to hold your foot to the floor to clean it out then it would start. I fixed them by takin the carb off cleaning the spots and then JB Weld over the plugs. Not sure of this is your issue but it sounded fimilar so thought I would throw that out there.
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Old 10-20-2020, 11:58 AM   #21
Rich72C10
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Re: Hard Starting Hot

I hope that’s not it, I paid some nice coin for this carb! But if / when I have to take it off I will double check that LNP. Thank you for your post!
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Old 10-20-2020, 02:16 PM   #22
Rich72C10
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Re: Hard Starting Hot

Okay a worse problem that we had hoped we had solved when the distributor nut was loose and new fuel pump was replaced (yesterday morning). Not sure if this is related to "Hot starting".

Intermittent severe missing or hesitation which dmjlambert reported and fixed when he had his carb sent off to National Carburetor. His posting seemed very similar to my issue I am fighting now. Though he didn't report any "Hot Starting" issues.

This morning we also went from 12 to 16 timing, for the record.

After we finish the work above we did nice early evening/dusk test drive (6:30pm or so). Some good highway speeds, 50~70 with a little bit of 80mph. Then city driving (lots of stops and goes). We stopped at the Kilgore Chevy/GM house since they had a C10 in the showroom (but it seemed a bit frankensteined to me). We left the truck idling for at least 15 minutes while peaking into the showroom. Then the last bit of our drive back to my brother's house was farm roads (no more than 50mph but lots of 30s). So this trip was roughly 50 miles. No problems. Personally I think it was because my brother was in the truck.

This morning as I said a bit ago, we went with a 16 degree timing. I also needed to fix my left turn canceling (silly spring needed adjustment) but that let me get my steering wheel sorted as it was a few teeth off to the right and it was really getting on my nerves LOL

Soooo, I decided okay let me drive home to Austin, 11am I get no more than 8 miles after getting to 60 and I started getting missing or hesitation. This is actually what it was doing to me my first two tries going home yesterday but a lot worse (yesterday). But the common between yesterday and today is it was hot sun barreling down on my hood, where-as last nights perfect drive was no sun dusky cool.

When my brother gets back from a little trip he wants to put back on the Carter Qjet that the previous owner put on (parts store sold him a 78 Chevy Truck carb). He thought it had a flooding problem early on this rebuild and I bought that matching numbers 72 Chevy Carb from uscarburetors. I have yet to send the core back.

To anyone that took the time to read all that rambing - ideas?

Could this also be related to percolating / vapor lock / fuel boiling? The hard fuel lines to really close the the passenger's ram head. Then the hard line gets close to the block too when it comes up from the fuel pump.
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Old 10-20-2020, 02:37 PM   #23
geezer#99
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Re: Hard Starting Hot

Could be fuel.
Did you check the timing first. It moved before. Coulda moved again.
They slowly move retarded (cw).
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Old 10-20-2020, 03:02 PM   #24
Rich72C10
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Re: Hard Starting Hot

I checked the timing again after this mornings attempt. It’s solid 16 degrees.
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Old 10-20-2020, 03:27 PM   #25
Rich72C10
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Re: Hard Starting Hot

Quote:
Originally Posted by j_cst_10 View Post

I've worked on a handful of engines that have had this issue. Let me know what you find! If it is this I can tell you things to check and ways to fix it
btw, I did not miss your posting j_cst_10 - we are not seeing anything pour/dribble out of the primaries. Not sure if my photo from the top was of any use but that was a photo right after stopping the engine.
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