340-346 conversion issue

[GWHP]

Just completed a 340-346/353 conversion. Meteor piston and jug, ported muffler, no other porting. Left the carb but did a rebuild. Replaced fuel line and added an echo tank breather.

I'm having an issue with it bogging at high revs, also adjusting the H on the carb doesn't lower the rpms no matter how many turns.

It will rev up to 14500rpm for about 3 seconds and then bogs, if it idles for a little it will be able to do another short high rev before it bogs. It starts third pull and will idle all day.

I'm assuming it's a fuel delivery issue. I did carve a little groove out of the riser for the impulse but maybe it got blocked with gasket goop when I closed it up?

It also seems to build up pressure in the fuel tank, get a blow out when I open it after running.

Any thoughts?

 

[farminkarman]

View attachment 468545

Just completed a 340-346/353 conversion. Meteor piston and jug, ported muffler, no other porting. Left the carb but did a rebuild. Replaced fuel line and added an echo tank breather.

I'm having an issue with it bogging at high revs, also adjusting the H on the carb doesn't lower the rpms no matter how many turns.

It will rev up to 14500rpm for about 3 seconds and then bogs, if it idles for a little it will be able to do another short high rev before it bogs. It starts third pull and will idle all day.

I'm assuming it's a fuel delivery issue. I did carve a little groove out of the riser for the impulse but maybe it got blocked with gasket goop when I closed it up?

It also seems to build up pressure in the fuel tank, get a blow out when I open it after running.

Any thoughts?

Did you check the end gap on your ring before putting the cylinder on? If not, pull it down and check that. This will give you a chance to make sure your intake boot isn't screwed up as well.

 

[GWHP]

Did you check the end gap on your ring before putting the cylinder on? If not, pull it down and check that. This will give you a chance to make sure your intake boot isn't screwed up as well.

Hmm no I didn't, wouldn't this mostly effect compression? Intake boot was in good nick when I did the build.

 

[farminkarman]

Hmm no I didn't, wouldn't this mostly effect compression? Intake boot was in good nick when I did the build.

No, an excessive gap will hurt the impulse signal to the carb.

 

[cuinrearview]

My thoughts:
OEM parts?
Did you square off the riser?
Clearance the PTO side ribs on the case?
What bearings/seals/sealant on the bottom end? And where?

These go from easy to a nightmare and sometimes it's the OEM parts.

 

[Maintenance Chief]

Here's a 346xp hybrid I did that had the impulse blocked, easy fix but a PIA.

 

[farminkarman]

My thoughts:
OEM parts?
Did you square off the riser?
Clearance the PTO side ribs on the case?
What bearings/seals/sealant on the bottom end? And where?

These go from easy to a nightmare and sometimes it's the OEM parts.

On every single 350 I have built, the riser was nowhere near flat. Good call on that one.

 

[farminkarman]

I just looked back at the original post. The other thing you have to do on this conversion with a 346 cylinder is shorten the cylinder screws.

 

[GWHP]

OEM parts? Yes except the riser

Did you square off the riser? No I had no means to do this, gave it a basic measure with calipers and it seemed ok.

Clearance the PTO side ribs on the case? Yep got rid of all touching plastic

What bearings/seals/sealant on the bottom end? And where? I initially replaced the crank bearings with non OEM and sealed up the case. It ran fine although it didn't have the ported muffler then. I noticed the fly wheel moving horizontally while it was being revved and so broke it all down again and checked the bearings against OEM and they were about 2mm narrower and so the crank shaft was slipping side ways a fraction. I checked the old bearings and they actually seemed fine so gave em a good clean and greased them and put them back in. Permatex Motoseal is the gasket goop ive being using, I measured the squish and it was pretty bang on .020'' without a gasket. Used it one the case and then on top of the riser which is perhaps where I have accidentally blocked the impulse.

 

[GWHP]

Here's a 346xp hybrid I did that had the impulse blocked, easy fix but a PIA.

Yeah its very similar to how this is behaving, will run and cut great for one cut or so then start dieing. If I idle for about 15 seconds it will crank back up again and do another cut only to die again.

To add an extra wtf, if I open the gas tank cap and close it again it will crank back up straight away. Which is why I was chasing tank vent and fuel line issues for a bit. I've replaced the fuel lines and added a echo vent and also rebuilt the carb with a kit to try eliminate this.

Looks like I'm taking the top end off again!

 

[farminkarman]

@Shane¹ just recently went through some issues with a 350 that I had ported a cylinder for. Sounds like some of the things you found already were similar to what he had. Iirc, he found the ring had excessive end gap, and something wrong with how the intake boot/impulse nipple were seating on the cylinder.

 

[Shane¹]

@Shane¹ just recently went through some issues with a 350 that I had ported a cylinder for. Sounds like some of the things you found already were similar to what he had. Iirc, he found the ring had excessive end gap, and something wrong with how the intake boot/impulse nipple were seating on the cylinder.

Yes I first chased several air leaks then found the new ring somehow ended up being pretty high gap even though I feel like I checked it and it was new when I started. And I think what was causing the biggest issues is the intake boot was tight but as it would run and get hot the clamp and boot would almost just wiggle and loosen up letting it sound real crackly and lean then after idle it would go back to normal and before the cut was over did it all over.