7900 porting continued, pics.

[Wortown Mick]

Im interested in talking to the gurus about my project before I start to tackle the rest of it.

My idea was blueprinting the jug to the lower end and clearance grinding the bases of the transfer ports larger. The cutouts / ears on the lower end are considerably larger than the corresponding transfer ports they flow into.

I am most likely going to do some work with the ports as well with some minor widening & polishing.

Pics.

My templates a bit screwy on the unfinished side but its the second one, im using two and have what I need removed scribed as well.


Im wondering about the side profile of the transfers in relation to the piston stroke. IE: the large amount of material removed on the exhaust port side, should I profile the port round or square on the bore. Very easy idea thats very hard to explain. Im just thinking if I open it a little more, the intake charge might have a little more room moving through and around the piston.

I guess to explain it better , If you were looking out the wrist pin living inside it as a little engine gremlin. On the exhaust side would it be better to have the transfer opening on the bore shaped and smooth to curve towards the intake or open it up alot more, make the opening more square, yet still smooth inside? Ill have to take another pic.

Let me know what you think, especially those who have dealt with 7900's

 

[Wortown Mick]

Anyone?
Bueller?



108 views i figured id at least have somone tell me I crippled the jug by now

 

[blsnelling]

108 views i figured id at least have somone tell me I crippled the jug by now

Your jug's clippled. Sorry, but I've never ported one of these before.

 

[Wortown Mick]

Well, generally speaking 2 stroke wise removing that material and enlarging the transfers isnt a bad thing right?

I know 7900's are a different animal but not THAT different id hope.

 

[blsnelling]

If you're just talking about the lower transfers and matching the jug to the case, I can't see you going wrong there.

 

[Jacob J.]

I think you're doing fine too. You can feed the lower transfers on the 7900 and not risk losing anything. I think where people go wrong is enlarging the upper transfers too much and widening the intake and exhaust too much.

 

[Wortown Mick]

Alright good, I was thinking adding a few cc's of crankcase volume would almost certainly have a negative impact.

The tops of the transfers seem to be designed fairly well for scavenging. Tiny transfers but Im too afraid to modify them or the timing.

The compression always seemed low on this saw too, thats another matter though I do have some scoring on the piston skirt over the exhaust port.

How do I know when I need muriatic acid to clean up the bore?

 

[timberwolf]

If you feel aluminium stuck to the bore surface then acid can help. If you feel a scratch that is cut into the plating, then acid won't do much good. A metal dental pick can help determine if you have a scratch or aluminium deposit on the bore.

Opening up the lower transfers is likely not really going to help unless the rest of the porting is done such the saw flows more. You don't need to open the lower transfers up much or else you just add base volume and lower port velocity hurting scavenging. However sometimes if you go to longer intake durration and lose base compression you need larger transfer ducts to help make up for it, but that is more a race/piped saw thing.

IMHO you need to address the compression and bore issues you have before too much worry about porting.

 

[Wortown Mick]

IMO your right.

The bores not in too bad of shape. Nothing catches a fingernail .
The piston however is scored and catches a nail. It could be sanded for sure but a pistons not overly expensive either. It could definitely be cleaned up to a good useable level.

Ill have to check the compression & squish after I fix the saw up to reciprocating again. Its currently back to 64cc's just in case.

out of curiosity does filling with epoxy have any place in this mod?

 

[timberwolf]

Scrap the piston, new they are just tight enough, if it did enough rubbing to get scored it will be undersized, sanding will just make matters worse.

I have doubts about epoxy/JB holding up in a work saw. Having a chunk of JB weld working loose could be life ending to the motor. However JB seams to hold up well on race motors run for short periods.

Most closed port saws like the 7900 there have decent ducts so I don't see needing to fill anywhere. Some open port cylinders though could stand some filler in the back top corners of the transfers.

 

[blsnelling]

I'd listen to Timberwolf and replace that piston before I ran it again. You don't want to mess up your cylinder. If you have scoring on the piston, it's quite likely that you have aluminum transfered onto the bore. Get yourself some 400 grit wet dry paper and make sure that bore's clean. If there's any aluminum buildup, go to the hardware store and get yourself some Muratic acid. Use a Qtip to apply it. Let it set and bubble. Follow up with the sand paper. Use some kind of lube like penetrant when sanding and try to sand so that you're putting a criss-cross pattern in the bore.

 

[Wortown Mick]

Hm, I was just trying to visualize a way of returning the volume in areas less in the way of airflow.

Ill PM one of the dolmar dealers about a piston.

The worst part of the bore is where I goofed with the dremel.
:censored:

Its below where any contact would be made with the rings so I dont think I should overly concern myself with it, probably hit it with a little sandpaper then maybe ball hone the jug.

Ive been thinking I shouldve just left the damn saw together hahaha.

 

[Wortown Mick]

Thanks brad, your saying sand to leave cross hatching as opposed to vertically or horizontally.

Oh well, whole jugs only 200 if I mess it up ill call it an expensive lesson.

 

[Jacob J.]

Thanks brad, your saying sand to leave cross hatching as opposed to vertically or horizontally.

Oh well, whole jugs only 200 if I mess it up ill call it an expensive lesson.

Is this a used top end that you're modifying?

 

[Wortown Mick]

Well I bought it new, beat the snot out of it.

Noticed scoring on the exhaust skirt when i was doin my muffler mod so I pulled it apart. From there I noticed nothing lines up in the jug, everythings very mismatched and if I removed material id improve flow.. thats where im at now.

Im calling the scoring a result of poor warmup procedures. If there were more on the intake side id say its dolmars air non filters fault, every time I use the saw the intake boot gets covered in fines.

IMO the saws suppossed to comprete with the best offerings from the other 2 big mfgs, it wont untill they fix that damn filter, and not for an additional 80 dollars, maybe a recall like they should have been?