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Joined: Mon Sep 26, 2016 12:38 pm
Posts: 261
Vehicle: 1987 WT LWB Sierra

Post Posted: Tue Dec 26, 2017 11:30 pm 
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G'day all, just after some insight into why my car has trouble starting when hot, if it's a common issue, if there's something wrong etc etc.

For reference, it's 1987 SJ51 WT running a G13b (w/ extractors, if it matters).

Some background:

Cold starts are no worries - two pumps of the throttle, and it's literally running within two cranks. Easy. Idle is a little lumpy, but runs fine apart from that.

Hot starts however can be troublesome. If you turn it off, and start it again soon after, it's not a problem at all, say if you fill up at the servo and pay and come back. No worries. But if you pull up at the supermarket and go grocery shopping for half hour to an hour for example, then it'll crank for ages before starting. Sometimes even so much that the battery starts getting down and you can hear the starter motor note changing. And I've actually got a slightly larger battery than factory.

I suspected at first it was perhaps an issue of fuel evaporation in the carb which I've heard can happen in some older cars, and that somewhat explains why when cold after a few pumps of the the throttle and the accelerator pump primes it up, she's good.

But, the thing is, I've had the car a while now and so have had the chance to try a fair few different things, and nothing seems to help. For example, I've tried pumping the throttle before starting, holding the throttle flat while cranking, holding it halfway, nothing at all, just starting etc.

Could it be something to do with the auto choke, or some other automagic heat related mechanism on the carb?

Curious to see if it's common among sierras, or am I alone in this :shocked:

Cheers guys, I appreciate the insight!

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Joined: Sat Oct 06, 2012 6:19 pm
Posts: 1404
Vehicle: Sierras!! SWB and LWB

Post Posted: Wed Dec 27, 2017 1:39 am 
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I’ve seen it a bit with old cars and strangely enough planes.

I have no idea what causes it but the thought occurs to me that perhaps it’s the auto choke cooling down before the motor does. That’s only a guess!! Sorry, not exactly insightful

Perhaps see if you can replicate it with the carby hat off. Get it nice and warm on a drive, whip the hat off once your home and try to start it in half hour. See if the choke is activating.

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Joined: Thu Feb 26, 2009 11:00 pm
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Location: Melbourne

Post Posted: Wed Dec 27, 2017 10:14 am 
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G13b’s are all EFI.

Is it efi or carb?

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Joined: Tue Apr 20, 2010 12:00 am
Posts: 1932
Location: Georgetown, Guyana
Vehicle: 98 5 dr SQ420, 05 5dr JB420

Post Posted: Wed Dec 27, 2017 11:08 am 
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Gwagensteve wrote:
G13b’s are all EFI.


Widely believed but incorrect.

I have two G13Bs here - as defined by the engine number starting with the prefix G13B-nnnnnnn - where n is a number. One started life in a Swift GTi, the other in a Swift GLX with a twin choke Aisin carb.

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Joined: Fri Oct 29, 2010 2:23 pm
Posts: 5424
Location: Northcliffe, W.A.
Vehicle: LJs, Sierra, Jimny, Swift.

Post Posted: Wed Dec 27, 2017 11:30 am 
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Yeah my carby swift is a G13b. I've also pulled 2 carby 8v motors out of Sierras that were G13bs.

My Sierra does exactly the same thing regarding the hot start. It has a weber and that eliminates the factory phenolic spacer under the carb. I reckon it boils the fuel out of the bowl and fills the intake manifold with a super rich mixture which then has to be pumped though for the motor and replaced by an appropriate mixture before it will start.

The float bowl on mine is to hot to comfortably hold your hand on.

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Joined: Mon Sep 26, 2016 12:38 pm
Posts: 261
Vehicle: 1987 WT LWB Sierra

Post Posted: Wed Dec 27, 2017 4:07 pm 
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Ooops guys I think I done goofed with the engine. It is the standard sierra motor, carb. I wrote B accidentally. Sorry folks.

Any other ideas for some remedies?

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Joined: Mon Sep 26, 2016 12:38 pm
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Vehicle: 1987 WT LWB Sierra

Post Posted: Fri Dec 29, 2017 5:45 pm 
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Just thinking, if the fuel were to boil out of the bowl and thus take ages to pump it all through and start afresh, wouldn't that mean that the fuel is evaporating when driving on a hot day? I can't imagine that the factory design would allow that to happen, and my motor's all standard as far as I know, so I wouldn't think that something like that would be a cause, but I'm still not sure.


I did a small test the other day, where I warmed up the motor (not from driving, only sat it there idling until the temp gauge read where it normally does), then turned it off and filmed the cool-down process. See the video here:



The choke butterfly started moving around the 19 minute mark, then continued to close until 1 hour after shutdown, when it stopped. By the 30 minute mark the butterfly wasn't really closed much at all, and so I doubt that it would make a significant difference in a hot start, if that is what the problem is. I stopped the clock after 1:15, as it was obvious there was no more changes occurring. It started beautifully at that point, 1 or two cranks and she was purring. See video here:


Granted, the test probably wasn't very fair, considering the motor didn't REALLY get hot. The engine bay was still cold, the block was still mostly cold, but the coolant was hot and therefore so was the gauge. I'd like to do another test after a long drive when it's properly hot and see what happens there.

However, ignoring that, even with the motor partially hot (as it was in the test) the choke still took 20 minutes to even start moving, so I'd imagine if the motor and everything was properly hot, it'd take even longer, or at the very least take longer to fully close.

So, my question is, if the choke butterfly was only a tiny bit closed, how much of a difference would that make to a hot start? Also, when the motor does finally start, it runs normally. It can't get hot instantly and reset the choke, surely, but I'm not 100% sure how the choke mechanism works, I'll have to read deeper in my service manual and see what I can find.

Could there be anything else that could cause this behaviour? I'm really confused at this point, especially after my little test, and would really appreciate some outside perspectives.

Cheers guys!

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