Carb recommendation??

[steves86ta]

Hi Guys,

I got the new engine in the boat last weekend and headed out to Perris yesterday to get her broken in and see how she runs. I was very happy with my results, however I now believe the stock Qjet is too small, or just not ideal.

Here are the specs of the engine;
Vortec 350 (stock bore)
Custom cam (224/224, .503" at 1.5 on a 110* LSA installed at 108)
Stock vortec heads for now plan on replacing with Edelbrock E-Tec's
About 9.5:1 compression

As far as performance goes, the boat runs great. It pulled the 19 pitch prop I already have o 5100 easily and 56.9 mph via GPS. (I could have pulled more RPM if I trimmed out more, but I did not want to run the stock drive that high) I plan to "prop" it down to where WOT is about 4600.

With that being said, the engine really was a dog out of the hole especially trying to Ski. I changed to my 17 pitch prop to try to pull some skiers and it just has a very hard time when going from idle to WOT. Seriously bogged down and had a hard time building RPM from idle. My thinking is that it is either Carb related or Timing related. I have the initial timing set at 10* and it just seems like a huge bog.

So my question is that I want to get rid of the Qjet and switch over to a "traditional carb" I just can?t decide between a double pumper or a vacuum secondary. If this was a car I would go with a vacuum secondary but I do not know if a boat will run a vacuum secondary carb efficiently when you are at RPM all the time. I know most everyone recommends a double pumper when pushing a Jet, but I have an outdrive so would a double pumper still be the recommendation? Also what size I?m thinking no bigger than a 750.

Sorry for the long post, but thank you for any insight you can provide

Steve

 

[MBAKER]

Re: Carb recommendation??

What condition is the Qjet in? was it gone through when you put the new engine in. If not that would be a good place to start, could simply be a weak accelerator pump, causing it to lag on take off but once moving runs fine. When setup right and in good mechanical condition the Qjet actually run really well. Also any Qjet will be way more than enough carb for a 350.

If you want to switch you will have to get an adapter to fit your spreadbore intake, (or change intake) then you could put on a holley or edelbrock. I would suggest edelbrock simply because its the same as the weber merc used so all bracketry is available. I just put a edelbrock on mine and outside of the fuel line it was all store bought and bolt on. If you have a mechanical fuel pump you might even be able to find a factory fuel line that works, electric pump has to be custom. A 600cfm carb either way is as much as you need for that engine.

 

[steves86ta]

Re: Carb recommendation??

What condition is the Qjet in? was it gone through when you put the new engine in. If not that would be a good place to start, could simply be a weak accelerator pump, causing it to lag on take off but once moving runs fine. When setup right and in good mechanical condition the Qjet actually run really well. Also any Qjet will be way more than enough carb for a 350.

If you want to switch you will have to get an adapter to fit your spreadbore intake, (or change intake) then you could put on a holley or edelbrock. I would suggest edelbrock simply because its the same as the weber merc used so all bracketry is available. I just put a edelbrock on mine and outside of the fuel line it was all store bought and bolt on. If you have a mechanical fuel pump you might even be able to find a factory fuel line that works, electric pump has to be custom. A 600cfm carb either way is as much as you need for that engine.

The QJet that is on the boat is pretty much brand new. I also have one that is a fresh rebuild that i could place on the boat as well. Just from my car experience, ive never really looked at Quadrajets for throttle response they always seemed to lag until the secondaries opened up. but i will try out the fresh rebuild one that i have.

I dont need to worry about the intake as i have a performer intake which is set up to accept both square and spread bores

I did find an edelbrock locally for sale that is a 750 marine carb #1410 that im looking at picking up im just worried about performance between that and a "double pumper"/

I am already running a holley red marine pump so the fuel system should be more than adequate.

thanks for the info!

 

[MBAKER]

Re: Carb recommendation??

when you say it bogs is it when you first take off it hesitates, or when you take off and put it to WOT it starts to go then bogs when the secondaries open?

 

[steves86ta]

Re: Carb recommendation??

Its hard to explain...

If im at idle or low speed and i go WOT it just plain bogs down or really has no power (very hard time pulling a single skier up) however once the RPM builds to about 3k or so it then starts to pull pretty good and runs very well.

It sort-of feels like the secondaries are not opening because once she gets going then it starts to pull pretty hard. I equate it to the secondaries opening in an automotive application.

If that makes any sense

 

[MBAKER]

Re: Carb recommendation??

If you ease into from idle just to take off cruising, not pulling or anything what does it do?

When you say it bogs down what does it sound like..quiet like the engine is dieing, or does it make a "BBBBOOOOOOOGGGGGGGGGG" noise that evenutally goes away as rpms increase?

 

[steves86ta]

Re: Carb recommendation??

I guess the easiest way to explain it, would be that it just does not have enough fuel/air to build RPM's. It just sort of falls on its face until the RPM builds. I think you may have a good idea bout the accelerator pump, or maybe some pickup is damaged or something along those lines.

Im more of an EFI guy myself and carbs are old school to me. You can hear that the throttle is open it just doesnt build any power or RPM is slow to build.

If you ease into the throttle it does pretty decent, on a side note the motor did spin up to 5k relatively easy and if you left it at WOT it just continued to pull however i had to shut it down. The more i think about it it does make sense that it is something internal with the carb in regards to a fuel pickup of some sort. like it takes a while to fill up the carb if that makes sense.

 

[steves86ta]

Re: Carb recommendation??

Or it may just be not enough timing down low and it takes the advance to come in for it to build power

 

[MBAKER]

Re: Carb recommendation??

yeah id check timing just to rule that out.

Also make sure the spring tension on the secondary air door is NOt backed way off. The secondary butterflies are mechanical, but the air door is based on demand. So if you go to WOT with the lever it will open the secondaries no matter what, even off idle. Then when there is enough demand from the engine the air door starts to open. But if the air door is backed off and flops open before the engine needs it it will bog and hesitate until rpms build because it drops signal to the metering system, and there is no secondary accelerator pump like on some holleys to make up for it.

 

[Idlespeedonly]

Re: Carb recommendation??

aaa

 

[Maclin]

Re: Carb recommendation??

In my opinion, and as MBAKER describer, it sounds more like the secondaries are opening too fast too soon, and your engine is actually getting more "carb" than it can use until the rpm's "catch up". I also agree that any Qjet has enough CFM for that engine with proper tuning.

 

[steves86ta]

Re: Carb recommendation??

Thanks for the help guys, i have done some research and fiddling with the boat, and think i figured it out. I decided to lock the timing out at 30* and that has helped a lot. I also swapped over to the freshly rebuilt Qjet i have laying around and it runs better.

I have a friend with a marine double pumper that he will sell me for cheap really cheap (20 bucks) that is about 6 months old. Its a 600 and should work really well for my application.

He lives about 100 miles away so i have not picked it up yet but the only thing im worried about is the bowl hitting the thermostat housing. Does anyone know if this will fit? Im supposed to run and get it tonight i just dont want to take the trip if it wont fit :-/

thanks again
steve

 

[MBAKER]

Re: Carb recommendation??

I would be worried about running 30* locked out all the time in a boat leading to detonation in the lower midrange. Looking back I see your cam specs, is it possible the "bog" is just below where the cam comes in hard and it feels like lazy until you get some rpms built up because of that.


If its a typical chevy intake then you shouldnt have a problem as long as the thermostat housing does not extend backwards very much. You will have to adjust the fuel line obviously, and the throttle bracket.

 

[steves86ta]

Re: Carb recommendation??

We will be heading out to do some testing this next weekend and will keep an ear open for detonation.

On a side note i found the root cause of my issues...

I either
a. did not tighten the rocker nut and set the lash on the #1 intake valve (highly unlikely as i checked everything twice)
or
b. the rocker nut backed off... as i had no intake rocker on #1.

the weird part is it did not make any noise...

Fixed this and it runs much much better sounds better too.

As far as the carb goes, i know about the throttle bracket and fuel line. im just worried about the front bowl hitting the thermostat housing. worse comes to worse i can run a spacer.

i have an edelbrock performer intake so its not the factory GM manifold

thanks again

 

[MBAKER]

Re: Carb recommendation??

1 dead cylinder like that will make a big difference. Id prob fix that and try again before making too many changes. I just replaced a 305 that had a functional but very leaky intake valve on 1 cylinder and it killed all kinds of power. Acted very similar to what you are saying, bog or hesitation taking off chased through ignition and carb then finally did compression check and found it.

 

[steves86ta]

Re: Carb recommendation??

1 dead cylinder like that will make a big difference. Id prob fix that and try again before making too many changes. I just replaced a 305 that had a functional but very leaky intake valve on 1 cylinder and it killed all kinds of power. Acted very similar to what you are saying, bog or hesitation taking off chased through ignition and carb then finally did compression check and found it.

yes im pretty sure that it backed off, however i can not be 100%.

I have since bought new rocker nuts and replaced all of them to be on the safe side and also double and triple checked.

it still starts just as easily however it now sounds much lopier, and builds RPM much much faster. even sounds better so now its time for another trip out to the lake.

im still going to grab this double pumper and see what happens. I am very familiar with them and know how they work (as well as still have all of my jet kits and what not), plus the throttle response should be much better.

Ill make sure to let you know what happens

 

[MBAKER]

Re: Carb recommendation??

please do...

Id get the carb too...20$ is a no brainer if its in decent shape. Never know when you might need one.

 

[steves86ta]

Re: Carb recommendation??

please do...

Id get the carb too...20$ is a no brainer if its in decent shape. Never know when you might need one.

Well if you want an update here’s one lol

I got the double pumper on as well as a new distributor.

Headed out to Lake Elsinore this last Saturday and gave 'er hell. She ran perfectly. Gobs of torque and massive amounts of midrange. I tried both a 4 blade 24pitch Bravo 1 prop as well as the 19pitch 3 blade I had.


The 24 was just way to much prop. I was only able to run it to about 4200rpm and she just couldn’t spin anymore. Top speed I believe was around 57.

I threw the 19 pitch on and WOW talk about a huge difference. She ran hard, very hard. The engine was extremely responsive and pulled 5800 RPM no problem. The most impressive part to me was running around 30mph and hammering it and the engine instantly would shoot up to around the 5500 mark and we were doing 50+ pretty much instantly.

Now I did not want to spin this drive much higher than 5K for too long but had to see what we could do with the 19pitch prop since it was running so well. The torque was pretty insane and would actually push you into the seat pretty hard.

So at 5500 with the 19pitch we were able to run 59.9 on GPS however we backed out because the water was getting pretty rough. We toyed around rest of the morning and just relaxed. By the time we got ready to leave and wanted to do a couple more hard runs the batteries had died in my GPS so I have no clue what the true speed is. But it definitely felt faster. As well as the engine was pulling 5800. I still had some room to trim out but I really didn’t want to run the drive to 6k.

All in all the power this engine is now making is unreal and I can tell it’s getting a little healthier every time out as it breaks in. The plugs look perfect and the carb seems to be dialed in. I think a good solid 22-24 pitch 3 blade should push me into the mid to upper 60's maybe even 70 if I can get it to spin one to around the 5k mark.

I guess you could say I’m a happy camper and I will most definitely be heading out to the river sometime soon to get some good solid runs (with a bigger prop) and good batteries in the GPS. The regular speedo is pretty much worthless when I’m 55+ as all it does is bounce to the peg past 65 and back.

Thanks again for all your help!

 

[Volphin]

Re: Carb recommendation??

Nice cam choice!

 

[steves86ta]

Re: Carb recommendation??

Nice cam choice!

Thanks! The midrange on this cam is insane.