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Riddle Me This (Help With Carb Running Issue)


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#1 SuperDeLuxeNick

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Posted 23 May 2020 - 02:44 PM

Hi,hoping you can help.

 

1988 Mini City 998cc / Maniflow manifold / 1.75" RC40 / AAU needle / new coil, new leads, new points, new cap.

 

Have had the mini running now for approx 500 miles and 90% of the time no problem. Have altered the mixture a few times to get it a little smoother and this was achieved other than one every so often (especially when cold) its idle drops really low, runs lumpy and misfires occasionally and bog down. Park up and drive home after an hour and it runs great or will start running great 10 minutes into journey...

 

Yesterday, i set the timing and mixture etc from scratch. Started at 8deg and adjusted mixture. Ran well, no problems but really down on power. So re-timed but this time used the advance function and followed the instructions, advanced to approx 30degs @4000rpm, ran it down the road, the slightest bit of pinking underload so backed it off 2degs. Took it back out, and it flew! sweet as a nut, no flat spots anywhere - really pleased.

 

Today, take it out, let it get to temperature and it runs awful! Misfire under any sort of load, bogs massively, like its getting no fuel. Pulled over and richened the mixture about 5-6 sides of the nut and had to bring the fast idle up to stop it stalling. It ran better but was really burbling and still wasn't happy at low revs but it did run well with some accelerator.

 

Any idea what to check? Surely something is stuck / blocked? As it was set yesterday and was perfect. Now today i have altered quite a bit just to get it drivable... 

 

Many Thanks in advance.



#2 sonscar

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Posted 23 May 2020 - 03:39 PM

The float chamber could be flooding.Is the choke holding the jet out.Adjusting the carb when it has been running OK is probably not the answer.I would look for faults rather than adjustments.Good hunting,Steve..

#3 cal844

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Posted 23 May 2020 - 05:12 PM

Change the coil for a known working unit. Sounds electrical to me...

Did you oil the dash pot in the carb with the same that was in it previously?

#4 SuperDeLuxeNick

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Posted 23 May 2020 - 06:16 PM

thanks guys, yes the dashpot has proper SU oil in it - the coil is new and there is no starting issues at all and it did it with the previous coil too.



#5 cal844

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Posted 23 May 2020 - 06:46 PM

thanks guys, yes the dashpot has proper SU oil in it - the coil is new and there is no starting issues at all and it did it with the previous coil too.


Use a thicker oil, I use 20w50 engine oil in mine, the car doesn't run well on any other oil. It goes flat with no power and would randomly cut out.

#6 SuperDeLuxeNick

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Posted 24 May 2020 - 02:08 PM

Okay update, definitely a fuel issue. Reset it today, runs nicely, fuel filter fills up and constant fuel noted, yesterday I noticed that the filter was very low level. After a few mins of running, the filter then started to run dry again! So meaning it must be drawing air in somewhere? Or the pump is not working properly? Removing the fuel cap doesn’t alter. So next test is to remove pipes on pump and see if it’s pumping well? If so then it must be drawing air elsewhere, maybe carb?

#7 cal844

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Posted 24 May 2020 - 02:39 PM

Okay update, definitely a fuel issue. Reset it today, runs nicely, fuel filter fills up and constant fuel noted, yesterday I noticed that the filter was very low level. After a few mins of running, the filter then started to run dry again! So meaning it must be drawing air in somewhere? Or the pump is not working properly? Removing the fuel cap doesn’t alter. So next test is to remove pipes on pump and see if it’s pumping well? If so then it must be drawing air elsewhere, maybe carb?


Check all the fuel lines for perishing. Has the engine got a mechanical fuel pump? If so check your oil level and sniff it, if you smell petrol it needs a new pump.

I highly doubt it to be a carb issue.

#8 sonscar

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Posted 24 May 2020 - 02:46 PM

I agree with smelling the oil,my sons fuel pump failed letting fuel into the oil and cost me £2000.Steve..

#9 SuperDeLuxeNick

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Posted 24 May 2020 - 02:59 PM

Took the line off the pump side and it pumps loads of fuel into a jug. Reconnected and it goes back to trickling through into the filter. There is a small pipe on the tank side of the pump which is perished a bit so will change that once it’s cooled down

#10 SuperDeLuxeNick

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Posted 24 May 2020 - 03:00 PM

It is a mechanical pump

#11 SuperDeLuxeNick

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Posted 24 May 2020 - 03:04 PM

Oil smells like oil ha, no sign of fuel....

#12 SuperDeLuxeNick

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Posted 24 May 2020 - 03:04 PM

Again, took float chamber lid off and met with a lot of air pressure coming out...

#13 cal844

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Posted 24 May 2020 - 03:42 PM

Remove the fuel cap from the tank, is there any pressure released when you loosen the cap?

#14 Rorf

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Posted 24 May 2020 - 03:48 PM

Again, took float chamber lid off and met with a lot of air pressure coming out...

 

Looks like the air breather on the float chamber lid is blocked, there shouldn't be a AIR pressure build up there. Also when last was fuel filter changed.



#15 SuperDeLuxeNick

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Posted 24 May 2020 - 06:02 PM

Guys, thank you for the help :)

 

I popped a new filter in just now anyway, the previous was only 500 miles old.

 

no noticeable pressure noise when fuel cap comes off, maybe slightly.

 

Ah i didn't check the breather on the carb....

 

I have just renewed the frayed pipe, and checked it all through, once again ran the fuel pump into a jug and it chucks the fuel out...

 

connected it back up and it does seem to be running okay... but it was also running okay this morning then an hour later it wasn't.






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