PHM 40 popping and banging

welshmally · Post by **welshmally** » Mon Jun 13, 2011 12:52 pm

Hi there,

I have just got my '79 900SS going after a long break, which included a battle with the electrics!

Started first kick, but rear cylinder is popping and banging and running a bit cooler than front.

Good spark (Lucas RITA set to 34 deg adv) and I gave the PHM's a good clean out before reassembling.

I use a Carbtune II to balance but can only get anywhere near an even reading with the throttle, as it's impossible to get it to idle on the slide screws. Seems better (but not great) over 3000 rpm.

Pilot screws set to 1.5 turns as a starting point.

Any hints?

Thanks

Hugh

welshmally · Post by **welshmally** » Mon Jun 13, 2011 1:55 pm

Just as a matter of interest - and this may be completely irrelevant - the pilot jet screw in the rear cylinder does not move as easily as the front - it's quite 'lumpy' although does not appear to be damaged in any way.

Crashfangio · Post by **Crashfangio** » Tue Jun 14, 2011 1:23 am

Try turning pilot screw a bit (half to 1) turn further out. You may find that it is running too lean on the idle circuit. I had the same drama with my 32 mm Dellortos, 1.5 turns out is not far enough.

Ralph M · Post by **Ralph M** » Tue Jun 14, 2011 11:42 am

My Darmah had a similar problem on both pots - turned out that the thick rubber gaskets between the carbs and heads were perished allowing extra air in. Worth a quick visual check!

baddean · Post by **baddean** » Wed Jun 15, 2011 12:32 pm

Hey Hugh,
+1 on what both Crash and Ralph said. Good (cheap, easy) places to start.
The idle screws may need to be as much as 2.5-3 turns out depending other intake factors. And don't expect that both carbs will be set the same number of turns out either. They could well be a different number of turns out each and still be set to optimum for the individual carb.
If it is only better and not significantly better at 3000 rpms, then what Ralph said about the carb insulators is a good place to look. Depending on the length of the "break" you took from the bike, and if these are the original insulators, these could have deteriorated.
Also pull the slides out and look at the "ramp" that the idle screw rides on. It could be damaged or worn from the many times that the slide has banged back down on it to cause the idle screw to no longer raise the slide high enough to get a good idle. If they are damaged it is an easy (one time) fix to simply swap the slides from one carb to the other (turns them around) and use the fresh ramp on the other side of the slides. ( the slides have ramps on both sides).
Dean

welshmally · Post by **welshmally** » Wed Jun 15, 2011 3:08 pm

Thanks to all: the insulators are fairly new - less than 5 years old and with relatively low mileage in that time so probably not the cause. Also the exhaust gaskets are OK I think but a hard one to check for leaks.

Time to hit the pilot screws!!!

The weather forecast for tomorrow evening is appalling - may have to wait until the weekend to wheel her out of the garage

- this is Wales after all - second highest rainfall in the world after the Amazon Basin apparently!

Will be in touch

Hugh

welshmally · Post by **welshmally** » Thu Jun 16, 2011 3:12 pm

No rain yet - time to tinker........

I had a fiddle with the pilot screws - no difference at any setting, so it must be something else.

Checked the float heights...... both different and both (apparently) wrong.

Just to make sure that I am doing it right: with the carb flat on the inlet side, face down, the float height should be 17.5 - 18.5mm with the needle just located in the jet, not at the end of the play , i.e gentle pressure pushing the float up as far as it will go. (If the float is on the right, carb face down, jet closed by pushing float to the left as in the link below fig 13). Unless I am missing something, this looks correct.

Interestingly, the front was high (rich) and the rear was low (lean).... I think!

http://www.ducatimeccanica.com/dellorto ... o_3_2.html

Hugh

Bern · Post by **Bern** » Sat Jul 02, 2011 10:38 am

baddean wrote:Hey Hugh,
+1 on what both Crash and Ralph said. Good (cheap, easy) places to start.
The idle screws may need to be as much as 2.5-3 turns out depending other intake factors. And don't expect that both carbs will be set the same number of turns out either. They could well be a different number of turns out each and still be set to optimum for the individual carb.
If it is only better and not significantly better at 3000 rpms, then what Ralph said about the carb insulators is a good place to look. Depending on the length of the "break" you took from the bike, and if these are the original insulators, these could have deteriorated.
Also pull the slides out and look at the "ramp" that the idle screw rides on. It could be damaged or worn from the many times that the slide has banged back down on it to cause the idle screw to no longer raise the slide high enough to get a good idle. If they are damaged it is an easy (one time) fix to simply swap the slides from one carb to the other (turns them around) and use the fresh ramp on the other side of the slides. ( the slides have ramps on both sides).
Dean

I'm glad I read this. I just replaced the gaskets and o-rings and noticed that the ramps were worn as you indicated. It didn't dawn on me to swap the slides. I can take the slides off the too buy list..

Bern · Post by **Bern** » Sat Jul 02, 2011 11:07 am

welshmally wrote:Hi there,

I have just got my '79 900SS going after a long break, which included a battle with the electrics!

Started first kick, but rear cylinder is popping and banging and running a bit cooler than front.

Good spark (Lucas RITA set to 34 deg adv) and I gave the PHM's a good clean out before reassembling.

I use a Carbtune II to balance but can only get anywhere near an even reading with the throttle, as it's impossible to get it to idle on the slide screws. Seems better (but not great) over 3000 rpm.

Pilot screws set to 1.5 turns as a starting point.

Any hints?

Thanks

Hugh

Given that the rear is running cool and not hot it would lead me to believe that, if anything, the cylinder would be running rich. At 3000 I wouldn't think idle circuit is having much impact so it doesn't seem like the culprit.
Could the ignition be too far advanced on the rear cylinder? I'm not that familiar with the RITA as I have a dyna on mine. I had a similar problem with my front and the ignition was way too far advanced on the front. The plugs should tell the story if it's running too lean.

welshmally · Post by **welshmally** » Sun Jul 03, 2011 3:06 am

Thanks Bern,

carbs largely sorted out now - went for a first-time run yesterday to get the bike MOT'd (a UK 'roadworthyness' test we have to do every year). Popping quite a bit on the over run, but gave the exhaust nuts a good tweak and they were quite loose, so that was probably the culprit. Not sure if the spanner needs a whack with a mallet to ensure a snug fit, but they are as tight as I can get with a push

She idles at a steady 1200rpm with a nice crisp pick up, so the summer awaits!!!
H

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PHM 40 popping and banging

PHM 40 popping and banging

Re: PHM 40 popping and banging