Starting problem with Model 425 Mixer on a 1995 Chevy S10

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Frank
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Starting problem with Model 425 Mixer on a 1995 Chevy S10

Post by Frank »

One of our new members (floyd) isn't familiar with email and forums so I offered to start a thread on his behalf. I told him that the members here are much more knowledgeable than me and would be able to give him better advice. The following is transcribed from emails with him:
floyd wrote:Hi , my names Floyd , I’m new to your site/ business , I am terrible at computing and poor at posting on forums etc., so decided to try this even though I dislike email and don’t use it much , but I do love your propane forum. I have had many lpg vehicles over the years ( 40 yrs.)and I just had my 95 chev S10 pickup converted to lpg , ( should have done it myself) , not happy with the outcome, the installer used a 425 impco on it and it nearly totally drains my battery before starting , and that’s during the summer, it won’t start this winter for sure. Question , would a 225m be a better choice for cold weather starting and better fuel economy ? The s10 has the tbi injection ,is there an adaptor for this setup , would reducing the gap on the spark plugs help, would you have the parts available, thanks

The S10 has the 4.3 v/6 ,impco E converter ,vff lockoff , the converter is new and the lockoff rebuilt , the engine is in very good condition ,high oil pressure ran very well on gas.It appears that the 425 mixer air valve is not lifting while the engine is being cranked , seems to me that this engine is not drawing enough air for the big 425 mixer to open , of course once it starts to open fuel will flow and it does start , but it takes about 30 sec. for that to happen. thanks for going to the trouble of posting this Frank , looking forward to responses , maybe I’ll be able to figure out how to work this forum thing. thx, floyd
I asked floyd if he the conversion used any sort of electronics and if he was having a CEL and he replied
floyd wrote:hi frank , the s10 is on lpg only and only oem electronics. check engine light is on. the installer did install a start valve from the vaporizer and controlled 12v from existing gas fuel pump relay, I did check for voltage while trying to start engine and it was at about 3volts , quite ineffective , I’m not a believer in primers and neither is Impco , they state that a properly set up system wouldn’t require priming . I’m old school and hate electronics in any shape or form , I’ll take simplicity any day but I do need to get the s10 running ,so , whatever it takes. I do think the 225 mixer will work well as it is rated at 235 hp , the s10 is rated at 165hp , the 225 should provide better cold weather starting and possibly better fuel economy , I am certainly open to any suggestions though.

Frank
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Re: Starting problem with Model 425 Mixer on a 1995 Chevy S10

Post by Frank »

Since he already has the Model 425 and it seems to run OK once he can get it running, I thought that a simple and inexpensive way to get around the starting issue would be to use an SV (Start Assist Valve) or VPV (Vacuum Power Valve). The better but more expensive fix would be to use a Model 225 or 200.

Recently, I contacted Franz and he is enjoying retirement - he says "retirement is vastly underrated!". He also offered the following advice about this issue:
Franz wrote:The 425 is oversized for the 4.3. It might run at mid to full throttle but there is insufficient airflow in start mode to get the parts moving. Using the SAV or VPV is a hit or miss situation. He might find that the 200 will cure all the starting issues with minimal wide open throttle loss.

BigBlockMopar
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Re: Starting problem with Model 425 Mixer on a 1995 Chevy S10

Post by BigBlockMopar »

I would also try at looking at the engine's cranking speed.
A tired battery and/or worn starter might turn over the engine quite slowly.

I'm a believer in a new/fresh GEL-technology battery and a higher speed mini-starter.
The propane hose from vaporizer to 425 is about 80cm long but my car always starts within 1-3 seconds.
https://www.bigblockmopar.com
'73 Dodge Dart - 360ci - 11.3:1cr
MegaSquirt + HEI 7-pin timing control - Edelbrock AirGap - Cold Air Intake
IMPCO E / 425 mixer - A518 OD-trans - 3.55 gears - 225/50/17" tires.

Frank
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Re: Starting problem with Model 425 Mixer on a 1995 Chevy S10

Post by Frank »

According to Impco's catalog, their mixers require ~6" WC of internal vacuum to lift the air-gas valve off of its seat. While having a fresh battery and starter would certainly help to ensure that the engine cranks over quickly enough, it appears that Franz doesn't think that a 4.3L engine has enough cranking airflow for this to occur.

My understanding is that Impco's published nominal mixer ratings (CFM @ 1.5" or 2.0" Hg manifold depression) is when the air-gas valve reaches the top of its travel (ie, @ ~13.8" WC). I think it would equally useful to show how much cranking airflow is required to generate 6" WC at the air-gas valve. I've looked for this information but have not yet been able to find it.

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Re: Starting problem with Model 425 Mixer on a 1995 Chevy S10

Post by C3H8 »

The 4.3 is more then capable of generating the airflow for a 425 unless the 425 is worn internally. Hundreds of CA300/50 series mixers and 425 mixers were installed on UPS vans in Canada and worked successfully. If the only issue is starting then either an SV primer or an alternative high pressure primer will solve the issue as recommended in a previous post. Installing a 200 or 225 will not necessarily solve a starting issue as they use an air bypass for fuel mixture control. The 200/225 series mixers are manufactured with very lean settings from the factory and require primers in most cases anyways. It is true that the 425 is substantially larger than required for the 4.3, however unless it is exhibiting poor low end performance installing a 225 may not help. The main advantage of the 225 would be throttle response at low RPM. In reality the 425 is the better starting unit as it uses fuel bypass to enrichen idle mixtures rather then air bypass used on the 100/200/225 mixers.

Frank
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Re: Starting problem with Model 425 Mixer on a 1995 Chevy S10

Post by Frank »

I let Floyd know about the post to this topic and he replied with:
floyd wrote:Hi Frank ,good to hear from you. I have the S10 running ok , I-had removed the 425 and resealed it to the TBI thinking that it may be losing vacuum and not able to open the air valve , that was not the case , the 425. is just to big for the v/6, but because I need the vehicle for winter I installed a 12v primer on the converter and a momentary contact switch on the dash and that gets it started within 3-4 cranks.

There was also an issue that BigBlockMopar brought to my attention about my battery being weak and he was correct ,after installing a new one it did turn over a little faster and longer but still would not start as the air valve was not coming off it’s seat. the fuel economy is not as good as it should be, uses same amount of fuel as my chev 350 v/8 camper van, but as your source mentioned a 200 or 225 should do better for fuel economy , I have a new 225 but it’s an FB model , don’t believe that will work , would you know if changing the air valve would make it a non FB?

My other option is to go with a 300 model , there is an available adapter that will adapt the 300 directly to the TBI, I have got good fuel economy from chev 305 and 350 on straight lpg. thanks so much for your help and input on this Frank , I’ ll let you know the outcome of 225 or 300 install .Floyd. p.s. if you have either of the 2 mixers , let me know.
Since he has a feedback 225, then Floyd can either convert it to open loop with a new gas valve (PN AV1-12-2 or N-AV1-12-12) or add a feedback controller (like the the BlueCAT 100). I also have some limited quantities of Dual Curve PN 5952.

As for switching to the Model 300 instead, Gann also has a Model 300 to GM TBI throttle body adapter (Gann A9-31-2), which is still available. You can also use an adapter to mount the Model 300 directly to a Holley 4bbl throttle body (Impco A3-96 or Gann A9-15). The Impco A3-96 is still available but the Gann A9-15 is obsolete.

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