Tuning woes

Discuss mechanical problems here.
Forum rules
By using this site, you agree to our rules. Please see: Terms of Use
Post Reply
Mark Wilson
Minor Addict
Posts: 956
Joined: Fri Aug 26, 2011 12:42 pm
Location: West Yorkshire
MMOC Member: Yes

Tuning woes

Post by Mark Wilson »

I'm trying to set up my rebuilt 1275 Midget engine with an HS4 carb rebuilt with Burlen parts and modified to non Waxstat. I'm going to have it set up on a rolling road, as I need someone more competent than me with access to a range of needles, but I want to get it running smoothly enough for me to get it to the rolling road about forty miles away. I've got it running, with timing on the twist the dizzy till it idles fastest basis, and with the jet set at twelve flats down.

The problem is that I can't get it to run without choke even when fully warmed up. I've backed the fast idle screw off to take it out of the equation, but as soon as the choke goes in beyond the stage where it moves the jet the engine dies. Logically this tells me that it needs to be richer, but dropping the jet a few flats doesn't help and it sounds as if there is a bit of backfiring going on when I do. The plugs are very black, and it smells to be running very rich. As the choke is always in operation while it is running I can't use the lift the piston method of checking. (The carb doesn't have a lift pin, but I've removed the air filter and could lift it manually). I've removed and blanked off the vacuum advance.

I can't remember which needle I fitted - it was a recommendation on here. I'm going to strip the carb down tomorrow and check very carefully that I assembled it correctly, but does anyone have any suggestion as to what I may have done wrong?
kennatt
Minor Legend
Posts: 2625
Joined: Fri Feb 02, 2007 9:11 pm
MMOC Member: No

Re: Tuning woes

Post by kennatt »

Check needle not fully fitted into piston causing weak mixture at jet. Maybe?.
paul 300358
Minor Fan
Posts: 467
Joined: Thu May 11, 2017 8:23 pm
Location: South Cheshire
MMOC Member: Yes

Re: Tuning woes

Post by paul 300358 »

Possible air leak between carb and cylinder head?
Murrayminor
Minor Addict
Posts: 564
Joined: Mon Oct 15, 2018 9:47 am
MMOC Member: Yes

Re: Tuning woes

Post by Murrayminor »

Have you confirmed the timing is correct?
You mention backfiring, that usually points to a timing issue.
Proud owner of my first Morris Minor
paul 300358
Minor Fan
Posts: 467
Joined: Thu May 11, 2017 8:23 pm
Location: South Cheshire
MMOC Member: Yes

Re: Tuning woes

Post by paul 300358 »

Do you have a good spark at the plugs? Have you checked the oil in the dashpot?
Mark Wilson
Minor Addict
Posts: 956
Joined: Fri Aug 26, 2011 12:42 pm
Location: West Yorkshire
MMOC Member: Yes

Re: Tuning woes

Post by Mark Wilson »

Some good suggestions, thanks, but the saga continues...

I've now removed the carb & checked thoroughly, including refitting the choke linkage which was a bit sticky, and changing the needle (I thought the original was possibly not quite straight, although it moved ok). I've tightened the manifold and carb connections in case of any leakage there. I've tried rotating the dizzy to as far advanced and as far retarded as possible for it to keep running, and several positions between. (I can't check the static timing as I've got an Accuspark module, and my strobe, also Accuspark, has decided it doesn't want to work.)

None of this has changed the original problem of the engine running with choke and dying as soon as the choke is fully closed. The fast idle isn't involved as I've backed it off.

I'm thinking the problem lies in the carburettor rather than the timing. I'm hearing popping noises from the exhaust which I assume are from burning of unspent fuel, also spitting through the carb which (I think) was when it was over advanced. It seems to me that it is showing the signs of running rich while running with choke, but getting insufficient fuel once the choke is closed. So presumably the needle is shutting off fuel from the jet except when the jet is lowered. I've fitted the needle and small spring in the needle guide and fitted the guide flush with the bottom of the piston. Is there anything I could have done wrong in fitting the needle or the jet? I did renew the jet as part of the waxstat conversion. Below are a couple of photos from before the engine was fitted.

Any more ideas, however wacky, would be welcome!

Mark
Attachments
DSCF9957.JPG
DSCF9957.JPG (285.17 KiB) Viewed 826 times
DSCF9947.JPG
DSCF9947.JPG (203.49 KiB) Viewed 826 times
kennatt
Minor Legend
Posts: 2625
Joined: Fri Feb 02, 2007 9:11 pm
MMOC Member: No

Re: Tuning woes

Post by kennatt »

plug that tube in the carb body, I had a similar problem with an mgb and blanked off that tube,think its supposed to be attached to the manifold on mg.never really worked out what it was for.Put your finger over it and see what happens(unless you know whats its for and was attached to something and just missing in photo)
pgp001
Minor Addict
Posts: 667
Joined: Sun Aug 27, 2017 6:05 pm
Location: West Yorkshire
MMOC Member: Yes

Re: Tuning woes

Post by pgp001 »

That angled brass pipe is for connection to the engine breathing system, it goes to the oil trap canister on mine.
As Kennatt says, if left open it will affect the mixture and weaken it by allowing air in. I think it would probably only be noticeable at tickover though.

Phil
Mark Wilson
Minor Addict
Posts: 956
Joined: Fri Aug 26, 2011 12:42 pm
Location: West Yorkshire
MMOC Member: Yes

Re: Tuning woes

Post by Mark Wilson »

Well observed, but It has been connected to the canister since the engine was fitted. The vacuum advance was fitted but is now plugged.
Mark Wilson
Minor Addict
Posts: 956
Joined: Fri Aug 26, 2011 12:42 pm
Location: West Yorkshire
MMOC Member: Yes

Re: Tuning woes

Post by Mark Wilson »

Woo hoo 😊 Cracked it! I took a few days off the car and pondered, during which time Accuspark sent me a replacement timing light (brilliant service 👍)

The timing turned out to be far more advanced than I'd thought working from my dizzy clamp marks. Still wouldn't idle without any choke, so gradually wound the jet down. At about eight flats beyond the rule of thumb twelve flats it is running well enough for now.

Thanks for the many helpful contributions

Mark
Post Reply