High speed fuelling problems

[Davo]

What I first thought was an ignition problem, I've turned my thoughts to fuelling. I will be going along nicely at 60/70mph and suddenly it will stop; it will lose speed aggressively and drop 10 to 15 miles per hour and then behave normally again. Standard 1979 Dolly 1300 single carb.

My thoughts went: blockage, mixture, water in fuel.

I've dealt with the blockage possibility to a point; the gauze filter in the pump was and is fine, and from the pump to the carb the pipe is clear. I've also taken note of the Haynes manual method of clearing the jet which - as I'm sure a lot of you know - is opening the throttle wide and, with the air filter off, blocking the air, therefore creating a partial vacuum which sucks some crap out of the jet. I've not yet had the jet out to blow it through though, so the possibility of s##t in the jet is still there.

Mixture is - again according to the Haynes manual's method of setting - spot on. (Drove for 20+ miles, set to run at 1000rpm, lifted piston a bit, revs rose slightly.) I haven't had enough experience to set carbs by ear yet.

Water in fuel; is it worth draining the tank to eradicate this possibility? If so, can you drain it with the tank in situ? That is, the bung under the tank, can I access it from under the car?

It's a very irritating problem, because I can't cruise at anything above sixty on long journeys. Any suggestions will be gratefully received

[george]

drive it along until it falters switch of ignition (not all the way to prevent steering lock) and pull over remove a float top and see if theres the right fuel level in it if not then your pump is the suspect................. it does sound like pump

[Triumph1300]

Or
Remove fuel cap, try driving at problem speed, if it cures it, it's the fuel tank breather.

[richard4040]

My old 1300 used to do that. After I took it off the road we discovered that it could have been fuel evaporation.

[Davo]

Ah, there's a few things I hadn't thought of... Cheers!

drive it along until it falters switch of ignition (not all the way to prevent steering lock) and pull over remove a float top and see if theres the right fuel level in it if not then your pump is the suspect................. it does sound like pump

Could that not also make the float needle/seat suspect? I suppose it's a worthwhile change, only a tenner from Rimmers. It's had a new pump last July, but of course that doesn't make it immune to problems

[AndyThomas]

Check all your fuel lines from the tank to the pump are clear. You can blow from the pump back to the tank to clear any blockages. The rubber pipe joints can also cause blockages if they start to break down.

Many years ago I have a simular problem on my Toledo when they started put detergents into the fuel. This disloged all the c**p in the tank and blocked the lines. The car was ok to 50-60mph then just died. Blowing the lines out fixed the problem.

Andy.

[Jon Tilson]

Find a hill...accelarte hard up it..

If the car cuts out or looses power going up then its the fule pump or tank vacuum..The latter is easy to diagnose...just undo the tank cap and see if its under suction. Do this immediately after it cuts out.

I suspect its the pump but it could also be the coil. When my pupm was iffy it would just cut out after hard accelration adn would occasionally catch back up again if I backed right off.

The coil could be breaking down on weak mixture at cruise...the plug resistance is higher with weak mixture and coil load also goes up with higher revs, but I would expect more of a misfire than a complete cutting out.

You need a source of known good substitute parts...I have a few and I'm in West London.

Jonners

[trackerjack]

I have owned 1500 HLS and a sprint.
All the following will show as your symptoms.
A little crap in the needle valve on the carbs was on the HLS.
Over fueling with a sticky valve on the sprint and blocked cap breather.
and finally flakes in the tank are sucked up and block tank outlet and as soon as you stop are released so there appears to be no problem!(my sprint when I first got it, was burnt off by a diesel Peugeot )
All these have happened to me over the years.

[Davo]

If the car cuts out or looses power going up then its the fule pump or tank vacuum..The latter is easy to diagnose...just undo the tank cap and see if its under suction. Do this immediately after it cuts out.

Well I didn't do it immediately after it cut out, but a short time after I stopped the engine and undid the fuel cap, it was under suction. In that case, what do I have to do to remove the breather pipe? I'll blow it out or fit a new hose. I guess normal fuel flexi hose will do?

[Toledo Man]

Take the sender unit off the tank and shine a torch inside to see if there's any crap in the tank. I had this problem with my Dolomite 1300 when some bar-steward put sugar in the tank. I removed the tank, (4 bolts once the filler and outlet had been disconnected) cleaned it out, flushed the fuel line and it was fine after that.

[Jon Tilson]

The tank has a vent hose running back to the fuel filler neck. Your cap should have a vent hole too. Try a different cap.
For a quick fix to see if thats it, just run an open piece of pipe from the tank vent tube to somewhere up high in the boot....obvioiulsy not long term and keep the end away from wires etc...you know the score...


Jonners

[Davo]

Hi guys,

It turned out to be the rubber pipe which runs from the tank to the main fuel line. It had deteriorated and collapsed inside. Thanks for all the suggestions though!