Dellorto problems

[tomb7949]

I purchased a 1973 GT750 that had worn out Amals. Back in the '70's I owned a 1975 GT750 that I had replaced the stock 30 mm Dellortos with 32 mm Dellortos. I kept the essentially unused 30 mm Dellortos for the last thirty years and thought they would be nice replacements for the worn-out Amals. I got the correct intake manifolds and installed the 30 mm Dellortos; but, the bike wouldn't start. It was running with the Amals but with a lot of back firing.

The 30 mm Dellortos are clean inside and out. The accellerator pump diaphram seem to be good in both carbs but I can't get the accellerator pumps to pump fuel. What should I check and/or replace?

[Peter Mille]

Stuck fuel filters?
Is there fuel coming into the bowls?

[tomb7949]

Yes, there is fuel in the float bowls.

[Peter Mille]

Is the float adjustment OK?

[tomb7949]

These carburators are unchanged from when they were on a new 1975 750 GT. At the time, the bike was running perfectly and I did not do anything to the carbs. I simply swapped them for a pair of 32 mm Dellortos.

[Craig in France]

The accellerator pump diaphram seem to be good in both carbs but I can't get the accellerator pumps to pump fuel. What should I check and/or replace?

Have you primed the pumps? Altho' I've never had this problem, I understand it can happen if the diaphragms dry out.

(Steve A. knows more about this than I), but I seem to recall you need to remove the brass valve on the top of the pump body.

Ciao

Craig in UK/France

[tomb7949]

I didn't know that one could prime the accellerator pumps. Is there anything other than removing the brass valve that is needed?

I would presume that removing the brass valve would allow fuel flow into the accellerator pump and; thus, bleed out the trapped air in the accellerator pump.

[wdietz186]

If my memory is correct there is a check ball in the bottom of the pickup tube for the pump. Take off the float bowl and the long brass thingy off to the side.Not the one with the O-ring[thats the choke jet] Unscrew and clean with some aerosol carb cleaner. While you are at it take the pump diaphram off and blow out the passage from the float bowl. And on the other side of the pump squirt through the delivery side of the pump. You can take out the pump jet and clean it too. If the carbs sat with fuel in them it is likley gum clogging the passage and jet. Oh and while you are in there take the other jets out and clean everything else too!

[tomb7949]

Thanks for the advice.

I cleaned the carbs when I put them on a shelf thirty years ago. There doesn't appear to be any deposits. However, I will go through the carbs making sure that all of the jets and passages are clean.

[BevHevSteve]

dood..........

30 years of sitting [?] means it is time to install new seal kits, float needle valves and pump diaphrams. The rubber definately is dry and your card will not function as it should....

Yes you prime the pump my putting your finger over the pump non return valve and pulling the throttle. You can do this all day long and your carb won;t prime though - the diaphram is too stiff from age.

[Peter Mille]

It should even run without the accelerator pumps (working) at all....!!
Are you sure the ignition is OK, because you mentioned heavy backfire before...!!

[tomb7949]

The ignition seems to work OK. The reason for the backfire that I mentioned was the original Amal carbs that were badly worn. The problem I am having now is because I have replaced to original worn out Amal carbs with 30 year old Dellortos. I think that Steve's suggestion to replace all the seals and accellerator diaphrams will probably fix my problem.